Method of computed tomography using fluorinated gas-filled lipid microspheres as contract agents

ABSTRACT

Novel gas filled microspheres useful as computed tomography (CT) contrast agents. The microspheres are prepared from a gas and/or a gaseous precursor, and one or more stabilizing compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 08/445,299, filed May19, 1995, now U.S. Pat. No. 5,874,062, which is a continuation-in-partof U.S. application Ser. No. 08/247,656, filed May 23, 1994, nowabandoned, and a continuation-in-part of U.S. application Ser. No.08/116,982, filed Sep. 7, 1993, now U.S. Pat. No. 5,456,900, which is adivision of U.S. application Ser. No. 07/980,594, filed Jan. 19, 1993,now U.S. Pat. No. 5,281,408, which is a division of U.S. applicationSer. No. 07/680,984, filed Apr. 5, 1991, now U.S. Pat. No. 5,205,290.

FIELD OF THE INVENTION

The present invention relates to compositions for computed tomography.More particularly, the present invention relates to compositions forcomputed tomography which comprise gas-filled microspheres.

BACKGROUND OF THE INVENTION

Computed tomography (CT) is a diagnostic imaging technique whichmeasures, in its imaging process, the radiodensity of matter.Radiodensity of matter is typically expressed in Hounsefield Units (HU).Hounsefield Units are a measure of the relative absorption of computedtomography X-rays by matter and is directly proportional to electrondensity. Water has arbitrarily been assigned a value of 0 HU, air avalue of -1000 HU, and dense cortical bone a value of 1000 HU.

Various tissues in the body possess similar densities. Difficulty hasbeen encountered in generating by CT visual images of tissues whichpossess similar densities and which are proximate each other. Forexample, it is difficult to generate separate CT images of thegastrointestinal (GI) tract and adjacent structures, including, forexample, the blood vessels and the lymph nodes. Accordingly, contrastagents have been developed in an attempt to change the relative densityof different tissues, and thereby improve the diagnostic efficacy of CT.

A commonly used contrast agent for computed tomography, particularly inconnection with scans of the GI tract for increasing the radiodensity ofthe bowel lumen, is barium sulfate. Barium sulfate increases electrondensity in certain regions of the body, and is classified as a "positivecontrast agents."

Currently available CT contrast agents, including barium compounds, suchas barium sulfate, suffer from various drawbacks. For example, theviability of CT agents is generally extremely sensitive toconcentration. If the concentration is too low, little contrast isobserved. If the concentration is too high, beam hardening artifactsresult and are observed as streaks in the CT images. In addition,difficulty is generally encountered in visualizing the bowel mucosa withthe currently available contrast agents.

Lipid compositions, for example, lipid emulsions and/or suspensions,have been formulated as contrast agents, particularly for the GI tract.Lipids inherently possess an electron density that is lower than water.Accordingly, lipid compositions are capable of decreasing electrondensity and are generally termed "negative contrast agents".

Lipid compositions are capable of providing enhanced visualization in CTscans. However, lipid-based contrast agents also suffer from variousdrawbacks. For example, compositions which comprise lipid alone aregenerally unpalatable which limits their use for oral applications. Inaddition, lipid compositions are typically expensive to formulate.Undesirable side effects can also be caused from the high concentrationsof lipid which are frequently used in the lipid-based contrast agents toachieve adequate negative contrast in certain regions of the body, forexample, the bowel lumen. Patients with pancreatitis, peptic or gastriculcers, irritable bowel disease, Crohn's disease, or colitis areespecially prone to such side effects. Furthermore, lipid-based contrastagents are typically perishable and thus possess a limited shelf-life.

Accordingly, new and/or better contrast agents for CT are needed. Thepresent invention is directed to this, as well as other, important ends.

BRIEF DESCRIPTION OF THE PRIOR ART

In U.S. Pat. No. 5,205,290 referred to above, there is disclosed lowdensity microspheres serving as contrast agents for computed tomography,which are composed of biocompatible synthetic polymers or copolymersprepared from monomers, such as acrylic acid, methacrylic acid,ethyleneimine, acrylamide, ethylene glycol, N-vinyl-2-pyrrolidone, andthe like. In a preferred synthesis protocol, the microspheres areprepared using a heat expansion process in which the microspheres, madefrom an expandable polymer or copolymer, contain in their void orcavity, a volatile liquid. The microspheres are then heated,plasticizing the microspheres and volatilizing the liquid, causing themicrospheres to expand to up to about several times their original size.When the heat is removed, the thermoplastic polymer retains at leastsome of its expanded shape. Microspheres produced by this process tendto be of particularly low density, and are thus said to be preferred.

Volatile liquids useful in the heat expansion process of U.S. Pat. No.5,205,290 include aliphatic hydrocarbons, such as ethane;chlorofluorocarbons, such as CCl₃ F; tetraalkyl silanes, such astetramethyl silane; as well as perfluorocarbons, such as those havingbetween 1 and about 9 carbon atoms and between about 4 and about 20fluorine atoms, especially C₄ F₁₀. It is said to be important that thevolatile liquid not be a solvent for the microsphere polymer orcopolymer; and that the volatile liquid should have a boiling point thatis below the softening point of the microsphere polymer or copolymer.

The stabilized gaseous precursor filled microspheres used as contrastmedia in the present invention are distinguishable from those of U.S.Pat. No. 5,205,290 in that they are not made from a polymer or copolymerby a heat expansion process, and are not, therefore, subject to the samelimitations which require that the volatile liquid not be a solvent for,and not have a boiling point below the softening point of, themicrosphere polymer or copolymer.

D'Arrigo, U.S. Pat. Nos. 4,684,479 and 5,215,680 disclose gas-in-liquidemulsions and lipid-coated microbubbles, respectively, which are stableand said to be useful in several fields, including as contrast agentsfor echocardiography, and in the ultrasonic monitoring of local bloodflow. However, there is no suggestion that these compositions would beuseful as contrast media for computed tomography.

Quay published application WO 93/05819 discloses that gases with high Qnumbers are ideal for forming stable gases, and that "microbubbles" ofthese gases are useful as contrast agents in ultrasound imaging.However, the disclosure is limited to stable gases, rather than theirstabilization and encapsulation, as in the present invention; althoughin a preferred embodiment described on page 31, sorbitol is used toincrease viscosity, which in turn is said to extend the life of amicrobubble in solution. Also, it is not an essential requirement of thepresent invention that the gas involved have a certain Q number ordiffusibility factor. Quay contains no suggestion that the gasmicrobubbles would be effective as a contrast medium for computedtomography.

Vanderipe published application WO 93/06869 also discloses the use ofbubbles of gases and gas mixtures, including perfluorocarbons, asultrasound imaging enhancement agents. Again, however, these gas bubblesare not encapsulated and there is no suggestion of their use as contrastmedia for computed tomography.

Lanza et al., published application WO 93/20802 discloses acousticallyreflective oligolamellar liposomes for ultrasonic image enhancement,which are multilamellar liposomes with increased aqueous space betweenbilayers or have liposomes nested within bilayers in a nonconcentricfashion, and thus contain internally separated bilayers. Their use inmonitoring a drug delivered in a liposome administered to a patient, isalso described. However, there is no suggestion that these liposomescould serve as contrast media for computed tomography.

Widder et al., published application EP-A-0 324 938 discloses stabilizedmicrobubble-type ultrasonic imaging agents produced fromheat-denaturable biocompatible proteins, e.g., albumin, hemoglobin, andcollagen. Again, however, use of such compositions as contrast media forcomputed tomography is not described.

There is also mentioned a presentation believed to have been made byMoseley et al. at a 1991 Napa, California meeting of the Society forMagnetic Resonance in Medicine, which is summarized in an abstractentitled "Microbubbles: A Novel MR Susceptibility Contrast Agent". Themicrobubbles which are utilized comprise air coated with a shell ofhuman albumin. The stabilized gas-filled microspheres of the presentinvention are not suggested, nor is their use as contrast media forcomputed tomography.

Tei et al., unexamined patent application disclosure SHO 63-60943discloses contrast agents for ultrasonic diagnosis comprising aperfluorocarbon emulsion with an emulsion particle size of 1 to 10 μm,in which the perfluorocarbon is preferably 9 to 11 carbon atoms and theemulsifier may be, for example, a phospholipid or a nonionic polymericsurfactant such as poly(oxyethylene)-poly(oxypropylene) copolymers. Theemulsion may be prepared by utilizing a mixer. There is no suggestion,however, that these perfluorocarbon emulsions would be suitable for useas contrast media in computed tomography.

Knight et al., U.S. Pat. No. 5,049,388 discloses small particle aerosolliposome and liposome-drug combinations for medical use, for example, assystems for delivering drugs to the respiratory tract by inhalation.However, there is no suggestion that these liposomes can be gaseousprecursor filled or that they might serve as contrast media for computedtomography.

SUMMARY OF THE INVENTION

The present invention is directed to a contrast medium useful forcomputed tomography imaging, said contrast medium comprising stabilizedgas and gaseous precursor filled microspheres, wherein the gas may be,for example, air or nitrogen, but may also be derived from a gaseousprecursor, for example, perfluoropentane, and the microspheres arestabilized by being formed from a stabilizing compound, for example, abiocompatible lipid or polymer. In certain preferred embodiments, thebiocompatible lipid comprises a phospholipid which is in the form of alipid bilayer. A contrast medium in accordance with the presentinvention comprises a substantially homogenous as well as surprisinglystable suspension of microspheres comprising gas and stabilizingcompound. A unique aspect of the present invention involves the use ofperfluorocarbon gases which are capable of maintaining the integrity,and thus, enhancing the stability, of the microspheres.

The present invention also concerns a method for preparing stabilizedgas-filled microspheres for use as computed tomography imaging contrastmedia, comprising the step of agitating an aqueous suspension of astabilizing compound, for example, a biocompatible lipid or polymer, sothat stabilized gas-filled microspheres result. Desirably, this step iscarried out at a temperature below the gel to liquid crystalline phasetransition temperature of the biocompatible lipid so as to achieve astabilized gas-filled microsphere product.

The present invention further pertains to a method of providing anenhanced image of an internal region of a patient comprising (i)administering to the patient one or more of the present contrast media,and (ii) scanning the patient using computed tomography imaging toobtain visible images of the involved regions.

Also encompassed by the present invention is a method for diagnosing thepresence of diseased tissue in a patient, especially in thegastrointestinal regions of the patient, comprising (i) administering tothe patient one or more of the present contrast media, and (ii) scanningthe patient using computed tomography imaging to obtain visible imagesof any diseased tissue in the region.

The present invention further relates to a method for preparing in situin the tissue of a patient a contrast medium for computed tomography,the contrast medium comprising gas-filled microspheres, comprising (i)administering to the patient gaseous precursor-filled microspheres, and(ii) allowing the gaseous precursor to undergo a phase transition from aliquid to a gas to provide the gas-filled microspheres.

All of the above aspects of the present invention can be carried out,often with considerable attendant advantage, especially with regard toease of ingestion by a patient, by using gaseous precursors to form thegas of the gas-filled microspheres. Once ingested, and upon gasformation in, for example, the gastrointestinal tract, expansion of thegaseous precursor causes an increase in the volume of the contrastmedium and impart low density to the gastrointestinal tract, therebyenhancing computed tomography imaging thereof. These gaseous precursorsmay be activated by a number of factors, but preferably are temperatureactivated, that is, they are activated by exposure to elevatedtemperature. Such gaseous precursors are compounds which, at a selectedactivation or transition temperature, change phases from a liquid to agas. Activation thus takes place by increasing the temperature of thecompound from a point below, to a point above, the activation ortransition temperature. Optionally, the contrast medium may furthercomprise a liquid fluorocarbon compound, for example, a perfluorocarbon,to further stabilize the microspheres. Preferably, the fluorocarbonliquid is encapsulated by the microspheres.

The present invention also relates to a method for preparing stabilizedgas or gaseous precursor filled microspheres for use as a computedtomography imaging contrast medium. The method comprises agitating anaqueous suspension of a lipid (that is, the lipid stabilizing compound),in the presence of a gas or gaseous precursor, resulting in gas orgaseous precursor filed microspheres. Desirably, agitation is carriedout at a temperature below the gel to liquid crystalline phasetransition temperature of the lipid to achieve a preferred product.

Where a gaseous precursor is used, the gaseous precursor filledmicrosphere composition is generally maintained at a temperature atwhich the gaseous precursor is liquid until administration to thepatient. At the time of administration the temperature may, if desired,be raised to activate the gaseous precursor to form a gas. The resultinggas filled microspheres are then administered to the patient.Alternatively, the gaseous precursor filled microspheres may, ifdesired, be administered without raising the temperature, and thegaseous precursor allowed to form a gas as a result of the naturallyelevated internal temperature of a patient. The composition may beagitated, if necessary, prior to administration.

The present invention further pertains to a method of providing anenhanced image of an internal region of a patient, especially an imageof the gastrointestinal region of said patient, said method comprising(i) administering to the patient the foregoing contrast medium, and (ii)scanning the patient using computed tomography imaging to obtain visibleimages of said region.

The present invention also encompasses a method for diagnosing thepresence of diseased tissue in a patient, especially in thegastrointestinal regions of said patient, said method comprising (i)administering to the patient the foregoing contrast medium, and (ii)scanning the patient using computed tomography imaging to obtain visibleimages of any diseased tissue in the region.

These and other aspects of the invention will become more apparent fromthe following detailed description when taken in conjunction with thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

It should be noted that, for purposes of making the drawings morereadily understood, only single bilayers are shown. In fact, themembranes which these drawings illustrate may be either monolayers,bilayers, oligolamellar, or multilamellar. Consequently, the figuresdescribed below should in no way be taken as limiting the presentinvention to microspheres whose envelope or skin is comprised of only asingle layer or bilayer of stabilizing compound.

FIG. 1 depicts the stabilization of a gas-filled lipid bilayermicrosphere with a perfluorocarbon that is proximate the hydrophobictails of the lipids.

FIG. 2 depicts the stabilization of a gas-filled lipid oligolamellarmicrosphere with a perfluorocarbon that is proximate the hydrophobictails of lipids in a monolayer that is located within a lipid bilayer.

FIG. 3 depicts the stabilization of a gas-filled lipid bilayermicrosphere with a perfluorocarbon that is proximate the interiorhydrophilic head groups of the lipids.

FIG. 4 depicts the stabilization of a gas-filled lipid bilayermicrosphere with a perfluorocarbon that is proximate the exteriorhydrophilic head groups of the lipids.

FIG. 5 depicts the stabilization of a gas-filled lipid monolayermicrosphere with a perfluorocarbon that is proximate the interiorhydrophobic tails of the lipids.

FIG. 6 depicts the stabilization of a gas-filled lipid oligolamellarmicrosphere with a perfluorocarbon that is proximate the hydrophobictails of lipids in a monolayer that is located outside of a lipidbilayer.

DETAILED DESCRIPTION OF THE INVENTION

As employed above and throughout the disclosure, the following terms,unless otherwise indicated, shall be understood to have the followingmeanings.

"Stabilized" refers to microspheres which are substantially resistant todegradation that is caused, for example, by the loss of structural orcompositional integrity in the walls of the microspheres and/or by theloss of any significant portion of the gas or gaseous precursor which isencapsulated within the microsphere.

"Lipid" refers to a synthetic, semisynthetic or naturally-occurringamphipathic compound which comprises a hydrophilic component and ahydrophobic component. Lipids include, for example, fatty acids, neutralfats, phosphatides, glycolipids, aliphatic alcohols and waxes, terpenesand steroids.

"Microsphere" refers to a small spherical entity which is characterizedby the presence of an internal void. Preferred microspheres areformulated from lipids, including the various lipids described herein.In any given microsphere, the lipids may be in the form of a monolayeror bilayer, and the mono- or bilayer lipids may be used to form one ormore mono- or bilayers. In the case of more than one mono- or bilayer,the mono- or bilayers are generally concentric. The lipid microspheresdescribed herein include such entities commonly referred to asliposomes, micelles, bubbles, microbubbles, and the like. Thus, thelipids may be used to form a unilamellar microsphere (comprised of onemonolayer or bilayer), an oligolamellar microsphere (comprised of abouttwo or about three monolayers or bilayers) or a multilamellarmicrosphere (comprised of more than about three monolayers or bilayers).The internal void of the microspheres may be filled with a liquid,including, for example, an aqueous liquid, a gas, a gaseous precursor,and/or a solid or solute material, as desired.

"Liposome" refers to a generally spherical cluster or aggregate ofamphipathic compounds, including lipid compounds, typically in the formof one or more concentric layers, for example, bilayers. They may alsobe referred to herein as lipid microspheres.

"Polymer" refers to molecules formed from the chemical union of two ormore repeating units. Accordingly, included within the term "polymer"are, for example, dimers, trimers and oligomers. In preferred form, theterm "polymer" refers to molecules which comprise 10 or more repeatingunits.

"Semi-synthetic polymer" refers to a naturally-occurring polymer thathas been chemically modified. Exemplary naturally-occurring polymersinclude, for example, polysaccharides.

"Patient" refers to animals, including mammals, preferably humans.

The present invention is directed, inter alia, to contrast mediacomprising stabilized gas filled microspheres which are basicallybubbles of very small diameter comprising a "skin" or "envelope" of astabilizing compound that surrounds or encloses a cavity or void filledwith liquid or gas. The stabilizing compound provides integrity to themicrosphere such that the microspheres exist for a useful period oftime. The stabilized microspheres are particularly suitable for use ascontrast agents for computed tomography (CT). In embodiments where thestabilizing compound comprises, for example, a lipid, the microspherespossess a lower electron density relative to water. This lower electrondensity imparts highly desirable properties to the contrast agents ofthe present invention, particularly with respect to CT imaging.

The stabilized microspheres of the present invention comprise a gasand/or a gaseous precursor. Any of the various biocompatible gas andgaseous precursors may be used in the gas and gaseous precursor filledmicrospheres of the present invention. Preferred gases are gases whichare inert and which are biocompatible, that is, gases which are notinjurious to biological function. Preferred gases also have a lowsolubility and diffusibility in aqueous media.

Moreover, it is possible to utilize a gas and a gaseous precursortogether. A unique and preferred aspect of the present invention resultsfrom the discovery that when a gaseous precursor, for example, aperfluorocarbon, is combined with a gas ordinarily used to make thestabilized microspheres of the present invention, microspheres areobtained having an added degree of stability not otherwise obtainablewith the gas alone. Thus, it is a preferred aspect of the invention toutilized gaseous precursors which can be activated, for example, uponexposure to elevated temperatures, to form stabilized microspheres inthe form, for example, of stable foams, which can be utilized aseffective low density contrast agents for computed tomography.

Stabilized microspheres made with gaseous precursors have severaladvantages. First, as the gases generated from gaseous precursors tendto be insoluble and relatively non-diffusible, these gases can bestabilized for use as contrast media for computed tomography. Becausethe gases are relatively stable, less stabilizing compound is necessarythan would be required for more soluble and diffusible gases, such asnitrogen or air. In general, a thicker walled skin or envelope ofstabilizing compound, for example, a thick walled microsphere, isnecessary to stabilize gases such as air or nitrogen. While thick walledmicrospheres filled with air, nitrogen or other gases can be used as CTcontrast agents, the thick walls of such microspheres raise theeffective density of the contrast medium, which may in turn limit theeffectiveness of the contrast medium. Furthermore, thick walledmicrospheres may be relatively unpalatable for oral ingestion, or may bedifficult to metabolize following intravenous injection. With thegaseous precursors used in the present invention, for example, aperfluorocarbon, the stabilizing compounds can be less rigid and theresulting microspheres can be thinner walled and easier to metabolize,yet still possess sufficient stabilizing compound to stabilize themicrosphere.

As is described in more detail further below, the stabilizedmicrospheres used in the present invention may be formed simply byagitation of the stabilizing compound in an aqueous environment and inthe presence of a gas and/or gaseous precursor. Where a gaseousprecursor is used, the gaseous precursor filled microsphere contrastmedium which has been prepared, before administration to a patient, isdesirably maintained at a temperature at which the gaseous precursor isliquid. At the time of administration, it can be pre-shaken and theningested as a pre-formed foam. Alternatively, the contrast medium can beingested as a suspension to form a foam in situ within, for example, thestomach and gastrointestinal tract of a patient. The bowel motilityserves to mix the gaseous precursor within the stabilizing compound andthe increase in temperature serves to form the gas filled microspherebased foam in situ within the bowel. A preferred embodiment described indetail further below involves incorporating a suitable viscositymodifying agent, for example, a natural and semi-natural gum, celluloseor synthetic polymer, for example, polyethyleneglycol. In the presenceof such a viscosity modifying agent and the stabilizing compound, thegas bubbles as they are generated are coated with these compounds andbecome stabilized through this coating process, whereby the contrastmedium of the present invention is formed.

Thus, the microspheres are formed from, or created out of, a matrix ofstabilizing compounds which permit the gas filled microspheres to beestablished and thereafter retain their size and shape for the period oftime required to be useful in computed tomography imaging. Thesestabilizing compounds include those which have a hydrophobic/hydrophiliccharacter which allows them to form bilayers, and thus microspheres, inthe presence of water. Thus, water, saline or some other water-basedmedium, often referred to hereafter as a diluent, is an important aspectof the stabilized gas and gaseous precursor filled microsphere contrastagents of the present invention, particularly in embodiments involvingmicrospheres which comprise bilayers.

The stabilizing compound may be a mixture of compounds which contributevarious desirable attributes to the stabilized microspheres. Forexample, compounds which assist in the dissolution or dispersion of thefundamental stabilizing compound have been found advantageous. The gas,which can be a gas at the time the microspheres are made, or can be agaseous precursor which, in response to an activator, such as elevatedtemperature, is transformed from the liquid phase to the gas phase. Thevarious aspects of the stabilized gas and gaseous precursor filledcontrast agents of the present invention will now be described, startingwith the gases and gaseous precursors.

Gases and Gaseous Precursors

The microspheres of the present invention are essentially stabilizedbubbles which encapsulate a gas and/or a gaseous precursor. The gasesand/or precursors thereto provide the compositions with increasednegative density. This increases their effectiveness as contrast agentsfor CT.

Preferred gases are gases which are extremely stable. The term stablegas, as used herein, refers to gases which are substantially inert andwhich are biocompatible, that is, gases which are not injurious tobiological functions and which will not result in any degree ofunacceptable toxicity, including allergenic responses and diseasestates. Preferred also are gases which have low solubility and/ordiffusibility in aqueous media. Gases, such as perfluorocarbons, areless diffusible and are relatively insoluble in aqueous media.Accordingly, they are easier to stabilize into the form of bubbles inaqueous media.

Preferable gases include those selected from the group consisting ofair, noble gases, such as helium, neon, argon and xenon, carbon dioxide,nitrogen, fluorine, oxygen, sulfur-based gases, such as sulfurhexafluoride and sulfur tetrafluoride, fluorocarbons, perfluorocarbongases, and mixtures thereof. Preferred gases are perfluorocarbon gases.Exemplary perfluorocarbon gases include, for example, perfluoromethane,perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutaneand mixtures thereof. Also preferred are mixtures of different types ofgases, such as a perfluorocarbon gas and another type of gas, such asoxygen. The gases discussed in Quay, published application WO 93/05819,including the high "Q" factor gases described therein, may be used also.The disclosures of Quay, published application WO 93/05819 areincorporated herein by reference in their entirety. In addition,paramagnetic gases and gases of isotopes, such as ¹⁷ O, may be used. Itis contemplated that contrast media which comprise these latter gasesmay also be used in connection with other diagnostic techniques, such asMagnetic Resonance Imaging (MRI).

Other gases, including the gases exemplified above, would be readilyapparent to one skilled in the art based on the present disclosure.

In certain particularly preferred embodiments, a precursor to a gaseoussubstance is incorporated in the microspheres. Such precursors includematerials which are capable of being converted to a gas in vivo.Exemplary precursors are materials which are liquids at room temperatureand which, after being administered to a patient, undergo a phasetransition to a gas in vivo. Preferably, the gaseous precursor isbiocompatible, and the gas produced in vivo is biocompatible also.Exemplary of suitable gaseous precursors are of the perfluorocarbons. Asthe artisan will appreciate, a particular perfluorocarbon may exist inthe liquid state when the microspheres are first made, and are thus usedas a gaseous precursor, or the perfluorocarbon may be used directly as agas. Whether the perfluorocarbon is used as a liquid or a gas generallydepends on its liquid/gas phase transition temperature, or boilingpoint. For example, a preferred perfluorocarbon, perfluoropentane, has aliquid/gas phase transition temperature or boiling point of 29.5° C.This means that perfluoropentane will be a liquid at room temperature(about 25° C.), but will become a gas within the human body, the normaltemperature of which (37° C.) is above the transition temperature orboiling point of perfluoropentane. Thus, under normal circumstances,perfluoropentane is a gaseous precursor. As a further example, there arethe homologs of perfluoropentane, namely perfluorobutane andperfluorohexane. The liquid/gas transition of perfluorobutane is 4° C.and that of perfluorohexane is 57° C. Thus, perfluorobutane ispotentially useful as a gaseous precursor, although more likely as agas, whereas perfluorohexane would likely be useful as a gaseousprecursor only because of its relatively high boiling point.

A wide variety of materials can be used as gaseous precursors in thepresent compositions. It is only required that the material be capableof undergoing a phase transition to the gas phase upon passing throughthe appropriate temperature. Suitable gaseous precursors include, forexample, hexafluoroacetone, isopropyl acetylene, allene,tetrafluoroallene, boron trifluoride, 1,2-butadiene, 2,3-butadiene,1,3-butadiene, 1,2,3-trichloro-2-fluoro-1,3-butadiene,2-methyl-1,3-butadiene, hexafluoro-1,3-butadiene, butadiyne,1-fluorobutane, 2-methylbutane, decafluorobutane, 1-butene, 2-butene,2-methyl-1-butene, 3-methyl-1-butene, perfluoro-1-butene,perfluoro-2-butene, 4-phenyl-3-butene-2-one, 2-methyl-1-butene-3-yne,butyl nitrate, 1-butyne, 2-butyne,2-chloro-1,1,1,4,4,4-hexafluorobutyne, 3-methyl-1-butyne,perfluoro-2-butyne, 2-bromo-butyraldehyde, carbonyl sulfide,crotononitrile, cyclobutane, methylcyclobutane, octafluorocyclobutane,perfluorocyclobutene, 3-chlorocyclopentene, perfluorocyclopentane,octafluorocyclopentene, cyclopropane, perfluorocyclopropane,1,2-dimethyl-cyclopropane, 1,1-dimethylcyclopropane,1,2-dimethylcyclopropane, ethylcyclopropane, methylcyclopropane,diacetylene, 3-ethyl-3-methyl diaziridine, 1,1,1-trifluorodiazoethane,dimethyl amine, hexafluorodimethylamine, dimethylethylamine,bis-(dimethylphosphine)amine, perfluorohexane, perfluoroheptane,2,3-dimethyl-2-norbornane, perfluorodimethylamine, dimethyloxoniumchloride, 1,3-dioxolane-2-one, 4-methyl-1,1,1,2-tetrafluoroethane,1,1,1-trifluoroethane, 1,1,2,2-tetrafluoroethane,1,1,2-trichloro-1,2,2-trifluoroethane, 1,1-dichloroethane,1,1-dichloro-1,2,2,2-tetrafluoroethane, 1,2-difluoroethane,1-chloro-1,1,2,2,2-pentafluoroethane, 2-chloro-1,1-difluoroethane,1,1-dichloro-2-fluoroethane, 1-chloro-1,1,2,2-tetrafluoroethane,2-chloro-1,1-difluoroethane, chloroethane, chloropentafluoroethane,dichlorotrifluoroethane, fluoroethane, hexafluoroethane,nitropentafluoroethane, nitrosopentafluoroethane, perfluoroethylamine,ethyl vinyl ether, 1,1-dichloroethane, 1,1-dichloro-1,2-difluoroethane,1,2-difluoroethane, methane, trifluoromethanesulfonylchloride,trifluoromethanesulfonylfluoride, bromodifluoronitrosomethane,bromofluoromethane, bromochlorofluoromethane, bromotrifluoromethane,chlorodifluoronitromethane, chlorodinitromethane, chlorofluoromethane,chlorotrifluoromethane, chlorodifluoromethane, dibromodifluoromethane,dichlorodifluoromethane, dichlorofluoromethane, difluoromethane,difluoroiodomethane, disilanomethane, fluoromethane, iodomethane,iodotrifluoromethane, nitrotrifluoromethane, nitrosotrifluoromethane,tetrafluoromethane, trichlorofluoromethane, trifluoromethane,2-methylbutane, methyl ether, methyl isopropyl ether, methyllactate,methylnitrite, methylsulfide, methyl vinyl ether, neopentane, nitrousoxide, 1,2,3-nonadecane-tricarboxylic acid-2-hydroxytrimethylester,1-nonene-3-yne, 1,4-pentadiene, n-pentane, perfluoropentane,4-amino-4-methylpentan-2-one, 1-pentene, 2-pentene (cis), 2-pentene(trans), 3-bromopent-1-ene, perfluoropent-1-ene, tetrachlorophthalicacid, 2,3,6-trimethylpiperidine, propane, 1,1,1,2,2,3-hexafluoropropane,1,2-epoxypropane, 2,2- difluoropropane, 2-aminopropane, 2-chloropropane,heptafluoro-1-nitropropane, heptafluoro-1-nitrosopropane,perfluoropropane, propene, hexafluoropropane,1,1,1,2,3,3-hexafluoro-2,3-dichloropropane, 1-chloropropane,chloropropane-(trans), 2-chloropropane, 3-fluoropropane, propyne,3,3,3-trifluoropropyne, 3-fluorostyrene, sulfur (di)-decafluoride (S₂F₁₀), 2,4-diaminotoluene, trifluoroacetonitrile, trifluoromethylperoxide, trifluoromethyl sulfide, tungsten hexafluoride, vinylacetylene and vinyl ether.

In certain preferred embodiments, a gas, for example, air or aperfluorocarbon gas, is combined with a liquid perfluorocarbon, such asperfluorooctylbromide (PFOB), perfluorodecalin, perfluorododecalin,perfluorooctyliodide, perfluorotripropylamine, andperfluorotributylamine.

The size of the microspheres can be adjusted, if desired, by a varietyof procedures including, for example, microemulsification, vortexing,extrusion, filtration, sonication, homogenization, repeated cycles offreezing and thawing cycles, extrusion under pressure through pores ofdefined size, and similar methods.

For intravascular use, the microspheres preferably have diameters ofless than about 30 μm, and more preferably, less than about 12 μm. Fortargeted intravascular use including, for example, binding to certaintissue, such as cancerous tissue, the microspheres can be significantlysmaller, for example, less than 100 nm in diameter. For enteric orgastrointestinal use, the microspheres can be significantly larger, forexample, up to a millimeter in size. Preferably, the microspheres aresized to have diameters between about 20 μm and 100 μm.

Tabulated below is a listing of a series of gaseous precursors whichundergo phase transitions from liquid to gas at relatively close tonormal human body temperature (37° C.) or below. Also listed in thetable are the sizes, in diameter, of emulsified droplets that would berequired to form a microsphere of a maximum size of about 10 μm.

                  TABLE 1                                                         ______________________________________                                        Physical Characteristics of Gaseous Precursors and                            Diameter of Emulsified Droplet to Form a 10 μm Microsphere                                                  Diameter (μm) of                                            Boiling        emulsified droplet                                    Molecular                                                                              Point          to make 10 micron                            Compound Weight   (° C.)                                                                          Density                                                                             microsphere                                  ______________________________________                                        perfluoro-                                                                             288.04   29.5     1.7326                                                                              2.9                                          pentane                                                                       1-       76.11    32.5     6.7789                                                                              1.2                                          fluorobutane                                                                  2-methyl-                                                                              72.15    27.8     0.6201                                                                              2.6                                          butane                                                                        (isopentane)                                                                  2-methyl-1-                                                                            70.13    31.2     0.6504                                                                              2.5                                          butene                                                                        2-methyl-2-                                                                            70.13    38.6     0.6623                                                                              2.5                                          butene                                                                        1-butene-3-                                                                            66.10    34.0     0.6801                                                                              2.4                                          yne-2-methyl                                                                  3-methyl-1-                                                                            68.12    29.5     0.6660                                                                              2.5                                          butyne                                                                        octafluoro-                                                                            200.04   -5.8     1.48  2.8                                          cyclobutane                                                                   decafluoro-                                                                            238.04   -2       1.517 3.0                                          butane                                                                        hexafluoro-                                                                            138.01   -78.1    1.607 2.7                                          ethane                                                                        ______________________________________                                         *Source: Chemical Rubber Company Handbook of Chemistry and Physics Robert     C. Weast and David R. Lide, eds. CRC Press, Inc. Boca Raton, Florida.         (1989-1990).                                                             

It is part of the present invention to optimize the utility of themicrospheres by using gases of limited solubility. Limited solubility,as used herein, refers to the ability of the gas to diffuse out of themicrospheres by virtue of its solubility in the surrounding aqueousmedium. A greater solubility in the aqueous medium imposes a gradientwith the gas in the microsphere such that the gas will have a tendencyto diffuse out of the microsphere. A lesser solubility in the aqueousmedium will decrease the gradient between the microsphere and theinterface such that the diffusion of the gas out of the microsphere willbe impeded. Preferably, the gas entrapped in the microsphere has asolubility less than that of oxygen, namely, 1 part gas in 32 partswater. See Matheson Gas Data Book, Matheson Company, Inc. (1966). Morepreferably, the gas entrapped in the microsphere possesses a solubilityin water less than that of air; and even more preferably, the gasentrapped in the microsphere possesses a solubility in water less thanthat of nitrogen.

Stabilizing Compounds

One or more stabilizing compounds are employed to form the microspheres,and to assure continued encapsulation of the gases or gaseousprecursors. Even for relatively insoluble, non-diffusible gases, such asperfluoropropane or sulfur hexafluoride, improved microspherepreparations are obtained when one or more stabilizing compounds areutilized in the formation of the gas and gaseous precursor filledmicrospheres. These compounds maintain the stability and the integrityof the microspheres with regard to their size, shape and/or otherattributes.

A wide variety of stabilizing compounds can be employed in the contrastmedia of the present invention. When combined with a gas and/or agaseous precursor, the stabilizing compounds are capable of promotingthe formation, and improving the stability, of the microspheres. Thestabilized microspheres of the present invention are substantiallyresistant to degradation as measured by the loss of microspherestructure or encapsulated gas or gaseous precursor for a useful periodof time. Typically, the microspheres are capable of retaining at leastabout 90 percent by volume of its original structure for a period of atleast about two or three weeks under normal ambient conditions, althoughit is preferred that this period be at least about a month, morepreferably, at least about two months, even more preferably, at leastabout six months, and more preferably, about a year, and still morepreferably about three years. Thus, the microspheres of the presentinvention possess long shelf-lives, even under adverse conditions,including elevated temperatures and pressures.

The stability of the microspheres of the present invention isattributable, at least in part, to the materials from which themicrospheres are made, and it is often not necessary to employadditional stabilizing additives, although it is optional, and sometimespreferred, to do so. Such additional stabilizing agents and theircharacteristics are explained in more detail below.

In preferred embodiments, the stabilizing compounds comprisebiocompatible lipid compounds and/or polymeric compounds, with lipidsbeing preferred. Preferably, the lipids or polymers are inert. Becauseof the ease of formulation, including the ability of producing themicrospheres just prior to administration, the microspheres can be madeconveniently on site.

Biocompatible Lipids

A wide variety of biocompatible lipids can be used as the stabilizingcompound. Suitable lipids include, for example, lysolipids,phospholipids, such as phosphatidylcholines with both saturated andunsaturated lipids including dioleoylphosphatidylcholine,dimyristoylphosphatidylcholine, dipentadecanoylphosphatidylcholine,dilauroylphosphatidylcholine, dipalmitoylphosphatidylcholine (DPPC) anddistearoylphosphatidylcholine (DSPC); phosphatidylethanolamines, such asdioleoylphosphatidylethanolamine, distearoylphosphatidylethanolamine anddipalmitoylphosphatidylethanolamine; phosphatidylserines;phosphatidylglycerols; phosphatidylinositols; sphingolipids, such assphingomyelin; glycolipids, such as ganglioside GM1 and GM2;glucolipids; sulfatides; glycosphingolipids; phosphatidic acid; palmiticacid; stearic acid; arachidonic acid; oleic acid; lipids bearingpolymers, including such polymers as piolyethyleneglycol, chitin,hyaluronic acid or polyvinylpyrrolidone; lipids bearing sulfonatedmono-, di-, oligo- or polysaccharides; cholesterols and cholesterolhemisuccinate; tocopherols and tocopherol hemisuccinate; lipids withether and ester-linked fatty acids; polymerized lipids; diacetylphosphate; dicetyl phosphate; stearylamine; cardiolipin; phospholipidswith short chain fatty acids (C₆ to C₈); synthetic phospholipids withasymmetric acyl chains, for example, a first acyl chain of C₆ and asecond acyl chain of C₁₂ ; ceramides; polyoxyethylene fatty acid esters,polyoxyethylene fatty alcohols, polyoxyethylene fatty alcohol ethers,polyoxyethylated sorbitan fatty acid esters, glycerol polyethyleneglycol oxystearate, glycerol polyethylene glycol ricinoleate, sterols,ethoxylated soybean sterols, ethoxylated castor oil,polyoxyethylene-polyoxypropylene polymers, and polyoxyethylene fattyacid stearates; sterol aliphatic acid esters including cholesterolsulfate, cholesterol butyrate, cholesterol isobutyrate, cholesterolpalmitate, cholesterol stearate, lanosterol acetate, ergosterolpalmitate, and phytosterol n-butyrate; sterol esters of sugar acidsincluding cholesterol glucuronide, lanosterol glucuronide,7-dehydrocholesterol glucuronide, ergosterol glucuronide, cholesterolgluconate, lanosterol gluconate, and ergosterol gluconate; esters ofsugar acids and alcohols including lauryl glucuronide, stearoylglucuronide, myristoyl glucuronide, lauryl gluconate, myristoylgluconate, and stearoyl gluconate; esters of sugars and aliphatic acidsincluding sucrose laurate, fructose laurate, sucrose palmitate, sucrosestearate, glucuronic acid, gluconic acid, accharic acid, and polyuronicacid; saponins, including sarsasapogenin, smilagenin, hederagenin,oleanolic acid, and digitoxigenin; glycerol dilaurate, glyceroltrilaurate, glycerol dipalmitate, glycerol and glycerol esters,including glycerol tripalmitate, glycerol distearate, glyceroltristearate, glycerol dimyristate, glycerol trimyristate; long chainalcohols of, for example, about 10 to about 30 carbon atoms, includingn-decyl alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol, andn-octadecyl alcohol; alkyl phosphonates, alkyl phosphinates and alkylphosphites; 6-(5-cholesten-3fl-yloxy)- 1-thio--D-galactopyranoside;digalactosyl-diglyceride;6-(5-cholesten-3β-yloxy)hexyl-6-amino-6-deoxy-1-thio-β-D-galactopyranoside;6-(5-cholesten-3β-yloxy)hexyl-6-amino-6-deoxy-1-thio-β-D-mannopyranoside;12-(((7'-diethylaminocoumarin-3-yl)carbonyl)methylamino)octadecanoicacid;N-[12-(((7'-diethylaminocoumarin-3-yl)carbonyl)methylamino)octadecanoyl]-2-aminopalmiticacid; cholesteryl(4'-trimethylammonio)butanoate;N-succinyldioleoylphosphatidylethanolamine; 1,2-dioleoyl-sn-glycerol;1,2-dipalmitoyl-sn-3-succinyl- glycerol;1,3-dipalmitoyl-2-succinylglycerol;1-hexadecyl-2-palmitoylglycerophosphoethanolamine; andpalmitoylhomocysteine.

Suitable lipid compounds include also lipids typically used to makemixed micelle systems, such as lauryltrimethylammonium bromide;cetyltrimethylammonium bromide; myristyltrimethylammonium bromide;alkyldimethylbenzylammonium chloride (where alkyl is, for example, C₁₂,C₁₄ or C₁₆); benzyldimethyldodecylammonium bromide/chloride;benzyldimethylhexadecylammonium bromide/chloride;benzyldimethyltetradecylammonium bromide/chloride;cetyldimethylethylammonium bromide/chloride; and cetylpyridiniumbromide/chloride.

Suitable lipids for use in the present compositions include also lipidscarrying a net charge, for example, anionic and/or cationic lipids.Exemplary cationic lipids include, for example,N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTMA);1,2-dioleoyloxy-3-(trimethylammonio)propane (DOTAP);1,2-dioleoyl-e-(4'-trimethylammonio)butanoyl-sn-glycerol (DOTB); andlipids bearing cationic polymers, such as polylysine and polyarginine.In general the molar ratio of cationic lipid to non-cationic lipid inthe microsphere may be, for example, 1:1000, 1:100, preferably, between2:1 to 1:10, more preferably in the range between 1:1 to 1:2.5 and mostpreferably 1:1 (ratio of mole amount cationic lipid to mole amountnon-cationic lipid, e.g., DPPC). A wide variety of lipids may comprisethe non-cationic lipid when cationic lipid is used to construct themicrosphere. Preferably, this non-cationic lipid isdipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine ordioleoylphosphatidylethanolamine. In lieu of the cationic lipids asdescribed above, lipids bearing cationic polymers, such as polylysine orpolyarginine, as well as alkyl phosphates, alkyl phophinates, and alkylphosphites, may also be used to construct the microspheres.

It has been surprisingly and unexpectedly found that the stability ofthe microspheres can be substantially improved by incorporating a smallamount, for example, about 1 to about 10 mole percent of the totallipid, of a negatively charged lipid. It is believed that the negativelycharged lipids enhance stability by reducing the tendency of themicrospheres to rupture by fusing together. It is believed that this isachieved, at least in part, by the formation of a negatively chargedlayer from the negatively charged lipid on the outer surface of themicrosphere. The negatively charged microsphere is then repulsed byother, similarly negatively charged microspheres. This repulsionprevents contact between microspheres which typically leads to a ruptureof the walls of the microspheres and consolidation of the contactingmicrospheres into larger microspheres.

Suitable negatively charged lipids include, for example, lipidscontaining free carboxy (CO₂ -) groups, such as phosphatidylserine,phosphatidic acid, such as dipalmitoylphosphatidic acid, and fattyacids. In certain preferred embodiments, the lipid comprisesdipalmitoylphosphatidylethanolamine and phosphatidic acid in a totalamount of from about 0.5 to about 30 mole percent. In certain otherpreferred embodiments, the lipid comprisesdipalmitoylphosphatidylcholine and distearoylphosphatidylcholine, in anamount of from about 70 to about 100 mole percent.

As noted above, it is desirable, in certain embodiments, to include asstabilizing compounds lipids bearing polymers. Preferably, the polymeris covalently bound to the lipid and has a molecular weight of fromabout 400 to about 100,000. Exemplary polymers include hydrophilicpolymers, such as poly(ethyleneglycol) (PEG), poly(vinylpyrrolidine),polyoxomers and polysorbate and poly(vinylalcohol). Preferred among thePEG polymers are PEG 2000, PEG 5000 and PEG 8000, which have molecularweights of 2000, 5000 and 8,000 respectively. Other suitable polymers,hydrophilic and otherwise, will be readily apparent to those skilled inthe art based on the present disclosure. Polymers which may beincorporated via alkylation or acylation reactions with a lipid areparticularly useful for improving the stability of the lipidcompositions. Exemplary lipids which bear hydrophilic polymers include,for example, dipalmitoylphosphatidylethanolamine-PEG,dioleoylphosphatidylethanolamine-PEG anddistearylphosphatidylethanolamine-PEG.

In addition to, or instead of, the lipid compounds discussed above, thepresent lipid compositions may comprise an aliphatic carboxylic acid,for example, a fatty acid. Preferred fatty acids include those whichcontain about 5 to about 22 carbon atoms in the aliphatic group. Thealiphatic group can be either linear or branched. Exemplary saturatedfatty acids include, for example, (iso)lauric, (iso)myristic,(iso)palmitic and (iso)stearic acids. Exemplary unsaturated fatty acidsinclude, for example, lauroleic, physeteric, myristoleic, palmitoleic,petroselinic, and oleic acid. Suitable fatty acids include also, forexample, fatty acids in which the aliphatic group is an isoprenoid orprenyl group. In addition, carbohydrates bearing polymers may be used inthe present lipid compositions. Carbohydrates bearing lipids aredescribed, for example, in U.S. Pat. No. 4,310,505, the disclosures ofwhich are hereby incorporated by reference herein, in their entirety.

Preferred lipids are phospholipids, including DPPC, DPPE, DPPA and DSPC,with DPPC being preferred.

Other lipid compounds for use in the present compositions, in additionto those exemplified above, would be apparent in view of the presentdisclosure. Preferably, lipids are selected to optimize certaindesirable properties of the compositions, including stability andhalf-life. The selection of suitable lipids in the preparation of thepresent compositions, in addition to the lipids exemplified above, wouldbe apparent to one skilled in the art and can be achieved without undueexperimentation, based on the present disclosure.

As discussed in detail below, a wide variety of methods are availablefor the preparation of microspheres including, for example, shaking,drying, gas-installation, spray drying, and the like. Preferably, themicrospheres are prepared from lipids which remain in the gel state,this being the temperature at which a lipid bilayer converts from thegel state to the liquid crystalline state. See, for example, Chapman etal., J. Biol. Chem. 1974 249, 2512-2521, the disclosures of which arehereby incorporated by reference herein, in their entirety. Thefollowing table lists representative lipids and their phase transitiontemperatures.

                  TABLE 2                                                         ______________________________________                                        Saturated Diacyl-sn-Glycero-3-Phosphocholines:                                Main Chain Phase Transition Temperatures                                      Carbons in   Main Phase Transition                                            Acyl Chains  Temperature ° C.                                          ______________________________________                                        1,2-(12:0)   -1.0                                                             1,2-(13:0)   13.7                                                             1,2-(14:0)   23.5                                                             1,2-(15:0)   34.5                                                             1,2-(16:0)   41.4                                                             1,2-(17:0)   48.2                                                             1,2-(18:0)   55.1                                                             1,2-(19:0)   61.8                                                             1,2-(20:0)   64.5                                                             1,2-(21:0)   71.1                                                             1,2-(22:0)   74.0                                                             1,2-(23:0)   79.5                                                             1,2-(24:0)   80.1                                                             ______________________________________                                    

See, e.g., Derek Marsh, CRC Handbook of Lipid Bilayers, p. 139 (CRCPress, Boca Raton, Fla. 1990).

The lipid material or other stabilizing compound used to form themicrospheres is also preferably flexible, by which is meant, in thecontext of gas and gaseous precursor filled microspheres, the ability ofa structure to alter its shape, for example, in order to pass through anopening having a size smaller than the microsphere.

Biocompatible Polymers

As noted above, the stabilizing compound can also comprise abiocompatible polymeric compound. The polymers can benaturally-occurring, semi-synthetic or synthetic. Exemplary naturalpolymers include, for example, polysaccharides, such as arabinans,fructans, fucans, galactans, galacturonans, glucans, mannans, xylans(such as, for example, inulin), levan, fucoidan, carrageenan,galactocarolose, pectic acid, amylose, pullulan, glycogen, amylopectin,cellulose, dextran, pustulan, chitin, agarose, keratin, chondroitan,dermatan, hyaluronic acid, alginic acid, xanthan gum, starch and variousother natural homopolymer or heteropolymers such as those containing oneor more of the following aldoses, ketoses, acids or amines: erythrose,threose, ribose, arabinose, xylose, lyxose, allose, altrose, glucose,mannose, gulose, idose, galactose, talose, erythrulose, ribulose,xylulose, psicose, fructose, sorbose, tagatose, mannitol, sorbitol,lactose, sucrose, trehalose, maltose, cellobiose, glycine, serine,threonine, cysteine, tyrosine, asparagine, glutamine, aspartic acid,glutamic acid, lysine, arginine, histidine, glucuronic acid, gluconicacid, glucaric acid, galacturonic acid, mannuronic acid, glucosamine,galactosamine, and neuraminic acid, and naturally occurring derivativesthereof.

Exemplary semi-synthetic polymers include carboxymethylcellulose,hydroxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose,and methoxycellulose.

Exemplary synthetic polymers include polyethylenes, such as, forexample, polyethylene glycol, polyoxyethylene, and polyethyleneterephthlate, polypropylenes such as, for example, polypropylene glycol,polyurethanes, such as, for example, polyvinyl alcohol (PVA),polyvinylchloride and polyvinylpyrrolidone, polyamides, such as, forexample, nylon, polystyrene, polylactic acids, fluorinated hydrocarbons,such as, for example, polytetrafluoroethylene, andpolymethylmethacrylate, and derivatives thereof. Methods for thepreparation of polymer-based microspheres will be readily apparent tothose skilled in the art, once armed with the present disclosure, andwhen coupled with information known in the art, such as the informationset forth in Unger, U.S. Pat. No. 5,205,290, the disclosures of whichare hereby incorporated by reference herein in their entirety.

Preferably, the polymer possesses a relatively high water bindingcapacity. When used, for example, in the GI region, a polymer having ahigh water binding capacity can bind large amounts of free water. Thisenables the polymer to carry a large volume of liquid through the GItract, thereby filling and distending the tract. The filled anddistended GI tract permits enhanced CT imaging of the region.

In addition, where imaging of the GI region is desired, the polymer ispreferably not substantially degraded in, and absorbed from, the GIregion. Thus, metabolism and absorption within the GI tract ispreferably minimized to avoid removal of the contrast agent. This alsoavoids the possible formation of gas within the GI tract from suchdegradation. For imaging the GI region, preferred polymers are capableof displacing air and minimizing the formation of large air bubbleswithin the contrast medium.

Particularly preferred embodiments of the present invention includemicrospheres wherein the stabilizing compound from which the stabilizedgas and gaseous precursor filled microspheres are formed comprises threecomponents: (1) a neutral lipid, for example, a nonionic or zwitterioniclipid, (2) a negatively charged lipid, and (3) a lipid bearing ahydrophilic polymer. Preferably, the amount of the negatively chargedlipid will be greater than 1 mole percent of total lipid present, andthe amount of lipid bearing a hydrophilic polymer will be greater than 1mole percent of total lipid present. It is also preferred that thenegatively charged lipid be a phosphatidic acid. The lipid bearing ahydrophilic polymer will desirably be a lipid covalently bound to thepolymer and the polymer will preferably have a weight average molecularweight of from about 400 to about 100,000. The hydrophilic polymer ispreferably selected from the group consisting of polyethyleneglycol(PEG), polypropyleneglycol, polyvinylalcohol, and polyvinylpyrrolidoneand copolymers thereof. The PEG or other polymer may be bound to alipid, for example, DPPE, through a covalent linkage, such as through anamide, carbamate or amine linkage. Alternatively, ester, ether,thioester, thioamide or disulfide (thioester) linkages may be used withthe PEG or other polymer to bind the polymer to, for example,cholesterol or other phospholipids. Where the hydrophilic polymer isPEG, a lipid bearing such a polymer can be referred to as being"PEGylated". The lipid bearing a hydrophilic polymer is preferablydipalmitoylphosphatidylethanolamine-PEG 5000 (DPPE-PEG 5000), whichmeans a dipalmitoylphosphatidylethanolamine lipid having a PEG polymerof a mean average molecular weight of about 5000 attached thereto.

Preferred embodiments of the present invention include microsphereswhich comprise, for example, about 77.5 mole percentdipalmitoylphosphatidylcholine 5 (DPPC), about 12.5 mole percent ofdipalmitoylphosphatidic acid (DPPA), and about 10 mole percent ofdipalmitoylphosphatidylethanolamine-PEG 5000. Such compositions, in aratio of mole percentages of 82:10:8 are preferred also. The DPPCcomponent is zwitterionic and therefore, effectively neutral, since thephosphatidyl portion is negatively charged and the choline portion ispositively charged. The DPPA component, which is negatively charged, isadded to enhance stabilization in accordance with the mechanismdescribed above regarding negatively charged lipids. The thirdcomponent, DPPE-PEG 5000, provides a PEGylated material bound to thelipid membrane or skin of the microsphere by the DPPE moiety, with thePEG moiety free to surround the microsphere membrane or skin, andthereby form a physical barrier to various enzymatic and otherendogenous agents in the body whose function is to degrade such foreignmaterials. It is also theorized that the PEGylated material is able todefeat the action of the macrophages of the human immune system, whichwould otherwise tend to surround and remove the foreign object. Theresult is an increase in the time during which the stabilizedmicrospheres can exist, in vivo, and therefore function as CT contrastagents.

Auxiliary Stabilizing Compounds

It is also contemplated to be a part of the present invention to preparestabilized gas and gaseous precursor filled microspheres using materialsin addition to the biocompatible lipids and polymers described above,provided that the microspheres so prepared meet stability and othercriteria set forth herein. These materials may be basic and fundamentaland thus, can form the primary basis for creating or establishing thestabilized gas and gaseous precursor filled microspheres. On the otherhand, they may be auxiliary, and therefore act as subsidiary orsupplementary agents which either enhance the functioning of the basicstabilizing compound or compounds, or else contribute some desiredproperty in addition to that afforded by the basic stabilizing compound.

However, it is contemplated that difficulty may be encountered indetermining whether a particular compound is a basic or an auxiliaryagent, since the functioning of the compound in question is generallydetermined empirically, or by the results produced with respect toproducing stabilized microspheres. For example, the simple combinationof a biocompatible lipid and water or saline, when shaken, will oftengive a cloudy solution subsequent to autoclaving for sterilization. Sucha cloudy solution may function as a contrast agent, but is aestheticallyobjectionable and may imply instability in the form of undissolved orundispersed lipid particles. Thus, propylene glycol may be added toremove this cloudiness by facilitating dispersion or dissolution of thelipid particles. The propylene glycol may also function as a thickeningagent which improves microsphere formation and stabilization byincreasing the surface tension on the microsphere membrane or skin. Itis possible that the propylene glycol further functions as an additionallayer that coats the membrane or skin of the microsphere, thus providingadditional stabilization.

Basic and auxiliary materials for use in the preparation of stabilizedmicrospheres would be apparent to one skilled in the art based on thepresent disclosure. Such materials include conventional surfactantswhich are disclosed, for example, in D'Arrigo, U.S. Pat. Nos. 4,684,479and 5,215,680, the disclosures of which are incorporated herein byreference, in their entirety.

Additional auxiliary and basic stabilizing compounds include such agentsas oils, for example, peanut oil, canola oil, olive oil, safflower oil,corn oil, or any other oil which is commonly known to be ingestible.Another auxiliary and basic stabilizing compound is trehalose.

It has been found that the gas and gaseous precursor filled microspheresused in the present invention may be controlled according to size,solubility and heat stability by choosing from among the variousadditional or auxiliary stabilizing agents described herein. Theseagents can affect these parameters of the microspheres not only by theirphysical interaction with the lipid coatings, but also by their abilityto modify the viscosity and surface tension of the surface of the gasand gaseous precursor filled microspheres. Accordingly, the gas andgaseous precursor filled microspheres may be favorably modified andfurther stabilized, for example, by the addition of a viscositymodifier, including, for example, carbohydrates and the phosphorylatedand sulfonated derivatives thereof, polyethers, including polyethershaving a molecular weight of, for example, from about 400 to about100,000 and di- and trihydroxy alkanes and their polymers having amolecular weight of, for example, about 200 to about 50,000; emulsifyingand/or solubilizing agents, including, for example, acacia, cholesterol,diethanolamine, glycerol monostearate, lanolin alcohols, lecithin, mono-and diglycerides, monoethanolamine, oleic acid, oleyl alcohol,poloxamer, polyoxyethylene 50 stearate, polyoxyl 35 castor oil, polyoxyl10 oleyl ether, polyoxyl 20 cetostearyl ether, polyoxyl 40 stearate,polysorbate 20, polysorbate 40, polysorbate 60, polysorbate 80,propylene glycol diacetate, propylene glycol monostearate, sodium laurylsulfate, sodium stearate, sorbitan monolaurate, sorbitan monooleate,sorbitan monopalmitate, sorbitan monostearate, stearic acid, trolamine,and emulsifying wax; suspending and/or viscosity-increasing agents,including, for example, agar, alginic acid, aluminum monostearate,bentonite, magma, carbomer 934P, carboxymethylcellulose, calcium andsodium and sodium 12, carrageenan, cellulose, dextran, gelatin, guargum, locust bean gum, veegum, hydroxyethyl cellulose,hydroxypropylmethylcellulose, magnesium-aluminum-silicate,methylcellulose, pectin, polyethylene oxide, povidone, propylene glycolalginate, silicon dioxide, sodium alginate, tragacanth, xanthan gum,α-d-gluconolactone, glycerol and mannitol; synthetic suspending agents,including, for example, polyethyleneglycol (PEG), polyvinylpyrrolidone(PVP), polyvinylalcohol (PVA), polypropylene glycol and polysorbate; andmaterials which raise the tonicity of the compositions, including, forexample, sorbitol, propyleneglycol and glycerol.

Aqueous Diluents

As mentioned earlier, where the microspheres are lipid in nature, aparticularly desired component of the stabilized microspheres is anaqueous environment of some kind, which induces the lipid, because ofits hydrophobic/hydrophilic nature, to form microspheres, which is ahighly stable configuration in such an environment. The diluents whichcan be employed to create such an aqueous environment include, but arenot limited to, water, either deionized or containing any number ofdissolved salts which will not interfere with the creation andmaintenance of the stabilized microspheres or their use as CT agents,and normal saline and physiological saline.

Methods of Preparation

The stabilized gas and gaseous precursor filled microspheres used in thepresent invention may be prepared by a number of suitable methods. Theseare described below separately for gas filled microspheres, gaseousprecursor filled microspheres, and both gas and gaseous precursor filledmicrospheres.

Methods of Preparation Using a Gas

A preferred embodiment comprises the steps of agitating an aqueoussolution containing a stabilizing compound, preferably a lipid, in thepresence of a gas at a temperature below the gel to liquid crystallinephase transition temperature of the lipid to form gas filledmicrospheres. The term agitating, and variations thereof, as usedherein, means any motion that shakes an aqueous solution such that gasis introduced from the local ambient environment into the aqueoussolution. The shaking must be of sufficient force to result in theformation of microspheres, particularly stabilized microspheres. Theshaking may be by swirling, such as by vortexing, side-to-side, or upand down motion. Different types of motion may be combined. Also, theshaking may occur by shaking the container holding the aqueous lipidsolution, or by shaking the aqueous solution within the containerwithout shaking the container itself.

Further, the shaking may occur manually or by machine. Mechanicalshakers that may be used include, for example, a shaker table such as aVWR Scientific (Cerritos, Calif.) shaker table, or a Wig-L-Bug® Shakerfrom Dental Mfg. Ltd., Lyons, Ill., which has been found to giveexcellent results. It is a preferred embodiment of the present inventionthat certain modes of shaking or vortexing be used to make stablemicrospheres within a preferred size range. Shaking is preferred, and itis preferred that this shaking be carried out using the Wig-L-Bug®mechanical shaker. In accordance with this preferred method, it ispreferred that a reciprocating motion be utilized to generate the gasand gaseous precursor filled microspheres. It is even more preferredthat the motion be reciprocating in the form of an arc. It is still morepreferred that the motion be reciprocating in the form of an arc betweenabout 2° and about 20°, and yet further preferred that the arc bebetween about 5° and about 8°. It is most preferred that the motion isreciprocating between about 6° and about 7°, most particularly about6.5°. It is contemplated that the rate of reciprocation, as well as thearc thereof, is particularly important in determining the amount andsize of the gas filled microspheres formed. Preferably, the number ofreciprocations or full cycle oscillations, is from about 1000 to about20,000 per minute. More preferably, the number of reciprocations oroscillations is from about 5000 to about 8000. The Wig-L-Bug®, referredto above, is a mechanical shaker which provides 2000 pestle strikesevery 10 seconds, i.e., 6000 oscillations every minute. Of course, thenumber of oscillations is dependent upon the mass of the contents beingagitated, with the larger the mass, the fewer the number ofoscillations. Another means for producing shaking includes the action ofgas emitted under high velocity or pressure.

It will also be understood that preferably, with a larger volume ofaqueous solution, the total amount of force will be correspondinglyincreased. Vigorous shaking is defined as at least about 60 shakingmotions per minute, and is preferred. Vortexing at about 60 to 300revolutions per minute is more preferred. Vortexing at about 300 to 1800revolutions per minute is even more preferred.

The formation of gas filled microspheres upon shaking can be detectedvisually. The concentration of lipid required to form a desiredstabilized microsphere level will vary depending upon the type of lipidused, and may be readily determined by routine experimentation. Forexample, in preferred embodiments, the concentration of1,2-dipalmitoylphosphatidylcholine (DPPC) used to form stabilizedmicrospheres according to the methods of the present invention is about0.1 mg/ml to about 30 mg/ml of saline solution, more preferably fromabout 0.5 mg/ml to about 20 mg/ml of saline solution, and even morepreferably from about 1 mg/ml to about 10 mg/ml of saline solution. Theconcentration of distearoylphosphatidylcholine (DSPC) used in preferredembodiments is about 0.1 mg/ml to about 30 mg/ml of saline solution,more preferably from about 0.5 mg/ml to about 20 mg/ml of salinesolution, and even more preferably from about 1 mg/ml to about 10 mg/mlof saline solution.

In addition to the simple shaking methods described above, moreelaborate methods can also be employed. Such elaborate methods include,for example, liquid crystalline shaking gas instillation processes andvacuum drying gas instillation processes, such as those described incopending U.S. application Ser. No. 08/076,250, filed Jun. 11, 1993,which is incorporated herein by reference, in its entirety. When suchprocesses are used, the stabilized microspheres which are to be gasfilled, may be prepared prior to gas installation using any one of avariety of conventional liposome preparatory techniques which will beapparent to those skilled in the art. These techniques includefreeze-thaw, as well as techniques such as sonication, chelate dialysis,homogenization, solvent infusion, microemulsification, spontaneousformation, solvent vaporization, French pressure cell technique,controlled detergent dialysis, and others, each involving preparing themicrospheres in various fashions. See, e.g., Madden et al., Chemistryand Physics of Lipids, 1990 53, 37-46, the disclosures of which arehereby incorporated herein by reference in their entirety.

The gas filled microspheres prepared in accordance with the methodsdescribed above range in size from below a micron to over 100 μm insize. In addition, it will be noted that after the extrusion andsterilization procedures, the agitation or shaking step yields gas andgaseous precursor filled microspheres with substantially no or minimalresidual anhydrous lipid phase in the remainder of the solution.(Bangham, A. D., Standish, M. M, & Watkins, J. C. (1965) J. Mol. Biol.Vol. 13, pp. 238-252 (1965). The resulting gas filled microspheresremain stable on storage at room temperature for a year or even longer.

The size of gas filled microspheres can be adjusted, if desired, by avariety of procedures, including microemulsification, vortexing,extrusion, filtration, sonication, homogenization, repeated freezing andthawing cycles, extrusion under pressure through pores of defined size,and similar methods. It may also be desirable to use the microspheres ofthe present invention as they are formed, without any attempt at furthermodification of the size thereof.

The gas filled microspheres may be sized by a simple process ofextrusion through filters; the filter pore sizes control the sizedistribution of the resulting gas filled microspheres. By using two ormore cascaded or stacked set of filters, for example, a 10 μm filterfollowed by an 8 μm filter, the gas filled microspheres can be selectedto have a very narrow size distribution around 7 to 9 μm. Afterfiltration, these stabilized gas filled microspheres remain stable forover 24 hours.

The sizing or filtration step may be accomplished by the use of a filterassembly when the suspension is removed from a sterile vial prior touse, or more preferably, the filter assembly may be incorporated intothe syringe itself during use. The method of sizing the microsphereswill then comprise using a syringe comprising a barrel, at least onefilter, and a needle; and will be carried out by a step of extractingwhich comprises extruding said microspheres from said barrel throughsaid filter fitted to said syringe between said barrel and said needle,thereby sizing said microspheres before they are administered to apatient in the course of using the microspheres as CT contrast agents inaccordance with the present invention. The step of extracting may alsocomprise drawing said microspheres into said syringe, where the filterwill function in the same way to size the microspheres upon entranceinto the syringe. Another alternative is to fill such a syringe withmicrospheres which have already been sized by some other means, in whichcase the filter now functions to ensure that only microspheres withinthe desired size range, or of the desired maximum size, are subsequentlyadministered by extrusion from the syringe.

In preferred embodiments, the solution or suspension of microspheres isextruded through a filter and is heat sterilized prior to shaking. Oncegas filled microspheres are formed, they may be filtered for sizing asdescribed above. These steps prior to the formation of gas and gaseousprecursor filled microspheres provide the advantages, for example, ofreducing the amount of unhydrated stabilizing compound, and thusproviding a significantly higher yield of gas filled microspheres, aswell as and providing sterile gas filled microspheres ready foradministration to a patient. For example, a mixing vessel such as a vialor syringe may be filled with a filtered stabilizing compound,especially lipid suspension, and the suspension may then be sterilizedwithin the mixing vessel, for example, by autoclaving. Gas may beinstilled into the lipid suspension to form gas filled microspheres byshaking the sterile vessel. Preferably, the sterile vessel is equippedwith a filter positioned such that the gas filled microspheres passthrough the filter before contacting a patient.

The first step of this preferred method, extruding the solution ofstabilizing compound through a filter, decreases the amount ofunhydrated compound by breaking up the dried compound and exposing agreater surface area for hydration. Preferably, the filter has a poresize of about 0.1 to about 5 μm, more preferably, about 0.1 to about 4μm, even more preferably, about 0.1 to about 2 μm, and still morepreferably, about 1 μm. Unhydrated compound, especially lipid, appearsas amorphous clumps of non-uniform size and is undesirable.

The second step, sterilization, provides a composition that may bereadily administered to a patient for CT imaging. Preferably,sterilization is accomplished by heat sterilization, preferably, byautoclaving the solution at a temperature of at least about 100° C., andmore preferably, by autoclaving at about 100° C. to about 130° C., evenmore preferably, about 110° C. to about 130° C., still more preferably,about 120° C. to about 130° C., and even more preferably, about 130° C.Preferably, heating occurs for at least about 1 minute, more preferably,about 1 to about 30 minutes, even more preferably, about 10 to about 20minutes, and still more preferably, about 15 minutes.

If desired, alternatively, the first and second steps, as outlinedabove, may be reversed, or only one of the two steps can be used.

Where sterilization occurs by a process other than heat sterilization ata temperature which would cause rupture of the gas filled microspheres,sterilization may occur subsequent to the formation of the gas filledmicrospheres, and is preferred. For example, gamma radiation may be usedbefore and/or after gas filled microspheres are formed.

Methods of Preparation Using a Gaseous Precursor

In addition to the aforementioned embodiments, one can also use gaseousprecursors contained in the microspheres which, upon activation, forexample, by temperature, light, or pH, or other properties of thetissues of a host to which it is administered, undergo a phasetransition from a liquid entrapped in the microspheres, to a gaseousstate, expanding to create the stabilized, gas-filled microspheres ofthe present invention. This technique is described in detail incopending patent applications Ser. No. 08/160,232, filed Nov. 30, 1993and Ser. No. 08/159,687, filed Nov. 30, 1993 both of which areincorporated herein by reference in their entirety.

The preferred method of activating the gaseous precursor is by exposureto elevated temperature. Activation or transition temperature, and liketerms, refer to the boiling point of the gaseous precursor which is thetemperature at which the liquid to gaseous phase transition of thegaseous precursor takes place. Useful gaseous precursors are thosematerials which have boiling points in the range of about -100° C. to70° C. The activation temperature is particular to each gaseousprecursor. An activation temperature of about 37° C., or about humanbody temperature, is preferred for gaseous precursors of the presentinvention. Thus, in preferred form, a liquid gaseous precursor isactivated to become a gas at 37° C. However, the gaseous precursor maybe in liquid or gaseous phase for use in the methods of the presentinvention.

The methods of preparing the CT imaging contrast agents of the presentinvention may be carried out below the boiling point of the gaseousprecursor such that a liquid is incorporated into a microsphere. Inaddition, the methods may be performed at the boiling point of thegaseous precursor such that a gas is incorporated into a microsphere.For gaseous precursors having low temperature boiling points, liquidprecursors may be emulsified using a microfluidizer device chilled to alow temperature. The boiling points may also be depressed using solventsin liquid media to utilize a precursor in liquid form. Further, themethods may be performed where the temperature is increased throughoutthe process, whereby the process starts with a gaseous precursor as aliquid and ends with a gas.

The gaseous precursor may be selected so as to form the gas in situ inthe targeted tissue or fluid, in vivo upon entering the patient oranimal, prior to use, during storage, or during manufacture. The methodsof producing the temperature-activated gaseous precursor-filledmicrospheres may be carried out at a temperature below the boiling pointof the gaseous precursor. In this embodiment, the gaseous precursor isentrapped within a microsphere such that the phase transition does notoccur during manufacture. Instead, the gaseous precursor-filledmicrospheres are manufactured in the liquid phase of the gaseousprecursor. Activation of the phase transition may take place at any timeas the temperature is allowed to exceed the boiling point of theprecursor. Also, knowing the amount of liquid in a droplet of liquidgaseous precursor, the size of the microspheres upon attaining thegaseous state may be determined.

Alternatively, the gaseous precursors may be utilized to create stablegas-filled microspheres which are pre-formed prior to use. In thisembodiment, the gaseous precursor is added to a container housing asuspending and/or stabilizing medium at a temperature below theliquid-gaseous phase transition temperature of the respective gaseousprecursor. As the temperature is then exceeded, and an emulsion isformed between the gaseous precursor and liquid solution, the gaseousprecursor undergoes transition from the liquid to the gaseous state. Asa result of this heating and gas formation, the gas displaces the air inthe head space above the liquid suspension so as to form gas-filledspheres which entrap the gas of the gaseous precursor, ambient gas (e.g.air), or coentrap gas state gaseous precursor and ambient air. Thisphase transition can be used for optimal mixing and stabilization of theCT imaging contrast medium. For example, the gaseous precursor,perfluorobutane, can be entrapped in the biocompatible stabilizingcompound, and as the temperature is raised, beyond 4° C., which is theboiling point of perfluorobutane, perfluorobutane gas is entrapped inmicrospheres. As an additional example, the gaseous precursorfluorobutane can be suspended in an aqueous suspension containingemulsifying and stabilizing agents, such as glycerol or propyleneglycol, and vortexed on a commercial vortexer. Vortexing is commenced ata temperature low enough that the gaseous precursor is liquid and iscontinued as the temperature of the sample is raised past the phasetransition temperature from the liquid to gaseous state. In so doing,the precursor converts to the gaseous state during themicroemulsification process. In the presence of the appropriatestabilizing agents, stable gas-filled microspheres result.

Accordingly, the gaseous precursors may be selected to form a gas-filledmicrosphere in vivo or may be designed to produce the gas-filledmicrosphere in situ, during the manufacturing process, on storage, or atsome time prior to use.

As a further embodiment of this invention, by pre-forming the gaseousprecursor in the liquid state into an aqueous emulsion, the maximum sizeof the microbubble may be estimated by using the ideal gas law, once thetransition to the gaseous state is effectuated. For the purpose ofmaking gas-filled microspheres from gaseous precursors, the gas phase isassumed to form instantaneously and substantially no gas in the newlyformed microsphere has been depleted due to diffusion into the liquid,which is generally aqueous in nature. Hence, from a known liquid volumein the emulsion, one would be able to predict an upper limit to the sizeof the gas-filled microsphere.

Pursuant to the present invention, an emulsion of a stabilizing compoundsuch as a lipid, and a gaseous precursor, containing liquid droplets ofdefined size may be formulated, such that upon reaching a specifictemperature, the boiling point of the gaseous precursor, the dropletswill expand into gas-filled microspheres of defined size. The definedsize represents an upper limit to the actual size because factors suchas gas diffusion into solution, loss of gas to the atmosphere, and theeffects of increased pressure are factors for which the ideal gas lawcannot account.

The ideal gas law and the equation for calculating the increase involume of the gas bubbles on transition from the liquid to gaseousstates is as follows:

    PV=nRT

where

P=pressure in atmospheres

V=volume in liters

n=moles of gas

T=temperature in ° K

R=ideal gas constant=22.4 L atmospheres deg⁻¹ mole⁻¹

With knowledge of volume, density, and temperature of the liquid in theemulsion of liquids, the amount (e.g. number of moles) of liquidprecursor as well as the volume of liquid precursor, a priori, may becalculated, which when converted to a gas, will expand into amicrosphere of known volume. The calculated volume will reflect an upperlimit to the size of the gas-filled microsphere, assuming instantaneousexpansion into a gas-filled microsphere and negligible diffusion of thegas over the time of the expansion.

Thus, for stabilization of the precursor in the liquid state in anemulsion wherein the precursor droplet is spherical, the volume of theprecursor droplet may be determined by the equation:

    Volume (sphere)=4/3 πr.sup.3

where

r=radius of the sphere

Thus, once the volume is predicted, and knowing the density of theliquid at the desired temperature, the amount of liquid (gaseousprecursor) in the droplet may be determined. In more descriptive terms,the following can be applied:

    V.sub.gas =4/3 π(r.sub.gas).sup.3

by the ideal gas law,

    PV=nRT

substituting reveals,

    i V.sub.gas =nRT/P.sub.gas

or,

    n=4/3[πr.sub.gas.sup.3 ]P/RT                            (A)

    amount n=4/3[πr.sub.gas.sup.3 P/RT]·MW.sub.n

Converting back to a liquid volume

    V.sub.liq =[4/3[πr.sub.gas.sup.3 ]P/RT]·MW.sub.n /D](B)

where D=the density of the precursor

Solving for the diameter of the liquid droplet,

    diameter/2=[3/4π[4/3·[πr.sub.gas.sup.3 ]P/RT]W.sub.n /D].sup.1/3

which reduces to

    Diameter=2[[r.sub.gas.sup.3 ]P/RT[MW.sub.n /D]].sup.1/3

As a further means of preparing microspheres of the desired size for useas CT imaging contrast agents in the present invention, and with aknowledge of the volume and especially the radius of the stabilizingcompound/precursor liquid droplets, one can use appropriately sizedfilters in order to size the gaseous precursor droplets to theappropriate diameter sphere.

A representative gaseous precursor may be used to form a microsphere ofdefined size, for example, 10 μm diameter. In this example, themicrosphere is formed in the bloodstream of a human being, thus thetypical temperature would be 37° C. or 310 K. At a pressure of 1atmosphere and using the equation in (A), 7.54×10⁻¹⁷ moles of gaseousprecursor would be required to fill the volume of a 10 μm diametermicrosphere.

Using the above calculated amount of gaseous precursor, and1-fluorobutane, which possesses a molecular weight of 76.11, a boilingpoint of 32.5° C. and a density of 0.7789 grams/mL⁻¹ at 20° C., furthercalculations predict that 5.74×10⁻¹⁵ grams of this precursor would berequired for a 10 μm microsphere. Extrapolating further, and armed withthe knowledge of the density, equation (B) further predicts that8.47×10⁻¹⁶ mL of liquid precursor are necessary to form a microspherewith an upper limit of 10 μm.

Finally, using equation (C), an emulsion of lipid droplets with a radiusof 0.0272 μm or a corresponding diameter of 0.0544 μm need be formed tomake a gaseous precursor filled microsphere with an upper limit of a 10μm microsphere.

An emulsion of this particular size could be easily achieved by the useof an appropriately sized filter. In addition, as seen by the size ofthe filter necessary to form gaseous precursor droplets of defined size,the size of the filter would also suffice to remove any possiblebacterial contaminants and, hence, can be used as a sterile filtrationas well.

This embodiment for preparing gas-filled microspheres used as CT imagingcontrast agents in the methods of the present invention may be appliedto all gaseous precursors activated by temperature. In fact, depressionof the freezing point of the solvent system allows the use gaseousprecursors which would undergo liquid-to-gas phase transitions attemperatures below 0° C. The solvent system can be selected to provide amedium for suspension of the gaseous precursor. For example, 20%propylene glycol miscible in buffered saline exhibits a freezing pointdepression well below the freezing point of water alone. By increasingthe amount of propylene glycol or adding materials such as sodiumchloride, the freezing point can be depressed even further.

The selection of appropriate solvent systems may be determined byphysical methods as well. When substances, solid or liquid, hereinreferred to as solutes, are dissolved in a solvent, such as water basedbuffers for example, the freezing point is lowered by an amount that isdependent upon the composition of the solution. Thus, as defined byWall, one can express the freezing point depression of the solvent bythe following equation:

    Inx.sub.a =In (1-x.sub.b)=ΔH.sub.fus /R(1/T.sub.o -1/T)

where:

x_(a) =mole fraction of the solvent

x_(b) =mole fraction of the solute

ΔH_(fus) =heat of fusion of the solvent

T_(o) =Normal freezing point of the solvent

The normal freezing point of the solvent results from solving theequation. If x_(b) is small relative to x_(a), then the above equationmay be rewritten:

    x.sup.b =ΔH.sub.fus /R[T-T.sub.0 /T.sub.0 T]≈ΔH.sub.fus ΔT/RT.sub.o.sup.2

The above equation assumes the change in temperature ⊖is small comparedto T₂. The above equation can be simplified further assuming theconcentration of the solute (in moles per thousand grams of solvent) canbe expressed in terms of the molality, m. Thus,

    X.sub.b m/[m+1000/m.sub.a ]≈mMa/1000

where:

Ma=Molecular weight of the solvent, and

m=molality of the solute in moles per 1000 grams.

Thus, substituting for the fraction x_(b) :

    ΔT=[M.sub.a RT.sub.o.sup.2 /1000ΔH.sub.fus ]m

    or ΔT=K.sub.f m, where

    K.sub.f =M.sub.alRT.sub.o .sup.2 /1000H.sub.fus

K_(f) is referred to as the molal freezing point and is equal to 1.86degrees per unit of molal concentration for water at one atmospherepressure. The above equation may be used to accurately determine themolal freezing point of gaseous-precursor filled microsphere solutionsused in the present invention. Hence, the above equation can be appliedto estimate freezing point depressions and to determine the appropriateconcentrations of liquid or solid solute necessary to depress thesolvent freezing temperature to an appropriate value.

Methods of preparing the temperature activated gaseous precursor-filledmicrospheres include:

(a) vortexing an aqueous suspension of gaseous precursor-filledmicrospheres used in the present invention; variations on this methodinclude optionally autoclaving before shaking, optionally heating anaqueous suspension of gaseous precursor and lipid, optionally ventingthe vessel containing the suspension, optionally shaking or permittingthe gaseous precursor microspheres to form spontaneously and coolingdown the gaseous precursor filled microsphere suspension, and optionallyextruding an aqueous suspension of gaseous precursor and lipid through afilter of about 0.22 μm, alternatively, filtering may be performedduring in vivo administration of the resulting microspheres such that afilter of about 0.22 μm is employed;

(b) a microemulsification method whereby an aqueous suspension ofgaseous precursor-filled microspheres of the present invention isemulsified by agitation and heated to form microspheres prior toadministration to a patient; and

(c) forming a gaseous precursor in lipid suspension by heating, and/oragitation, whereby the less dense gaseous precursor-filled microspheresfloat to the top of the solution by expanding and displacing othermicrospheres in the vessel and venting the vessel to release air; and

(d) in any of the above methods, utilizing a sealed vessel to hold theaqueous suspension of gaseous precursor and stabilizing compound such asbiocompatible lipid, said suspension being maintained at a temperaturebelow the phase transition temperature of the gaseous precursor,followed by autoclaving to move the temperature above the phasetransition temperature, optionally with shaking, or permitting thegaseous precursor microspheres to form spontaneously, whereby theexpanded gaseous precursor in the sealed vessel increases the pressurein said vessel, and cooling down the gas-filled microsphere suspension,after which shaking may also take place.

Freeze drying is useful to remove water and organic materials from thestabilizing compounds prior to the shaking gas instillation method.Drying-gas instillation methods may be used to remove water frommicrospheres. By pre-entrapping the gaseous precursor in the driedmicrospheres (i.e. prior to drying) after warming, the gaseous precursormay expand to fill the microsphere. Gaseous precursors can also be usedto fill dried microspheres after they have been subjected to vacuum. Asthe dried microspheres are kept at a temperature below their gel stateto liquid crystalline temperature, the drying chamber can be slowlyfilled with the gaseous precursor in its gaseous state, e.g.perfluorobutane can be used to fill dried microspheres composed ofdipalmitoylphosphatidylcholine (DPPC) at temperatures between 4° C. (theboiling point of perfluorobutane) and below 40° C., the phase transitiontemperature of the biocompatible lipid. In this case, it would be mostpreferred to fill the microspheres at a temperature about 4° C. to about5° C.

Preferred methods for preparing the temperature activated gaseousprecursor-filled microspheres comprise shaking an aqueous solutionhaving a stabilizing compound such as a biocompatible lipid in thepresence of a gaseous precursor at a temperature below the gel state toliquid crystalline state phase transition temperature of the lipid, andbelow the liquid state to gas state phase transition temperature of thegaseous precursor. Heating of the mixture to a temperature above theliquid state to gas state phase transition temperature of the gaseousprecursor then causes the precursor to expand. Heating is thendiscontinued, and the temperature of the mixture is then be allowed todrop below the liquid state to gas state phase transition temperature ofthe gaseous precursor. Shaking of the mixture may take place during theheating step, or subsequently after the mixture is allowed to cool.

The present invention also contemplates the use of a method forpreparing gaseous precursor-filled microspheres comprising shaking anaqueous solution comprising a stabilizing compound such as abiocompatible lipid in the presence of a gaseous precursor, andseparating the resulting gaseous precursor-filled microspheres forcomputed tomography imaging use. Microspheres prepared by the foregoingmethods are referred to herein as gaseous precursor filled microspheresprepared by a gel state shaking gaseous precursor instillation method.

Conventional, aqueous-filled liposomes of the prior art are routinelyformed at a temperature above the phase transition temperature of thelipids used to make them, since they are more flexible and thus usefulin biological systems in the liquid crystalline state. See, for example,Szoka and Papahadjopoulos, Proc. Natl. Acad. Sci. 1978, 75, 4194-4198.In contrast, the microspheres made according to preferred embodimentsdescribed herein are gaseous precursor filled, which imparts greaterflexibility, since gaseous precursors after gas formation are morecompressible and compliant than an aqueous solution. Thus, the gaseousprecursor filled microspheres may be utilized in biological systems whenformed at a temperature below the phase transition temperature of thelipid, even though the gel phase is more rigid.

The methods contemplated by the present invention provide for shaking anaqueous solution comprising a stabilizing compound, such as abiocompatible lipid, in the presence of a temperature activated gaseousprecursor. Shaking, as used herein, is defined as a motion that agitatesan aqueous solution such that gaseous precursor is introduced from thelocal ambient environment into the aqueous solution. Any type of motionthat agitates the aqueous solution and results in the introduction ofgaseous precursor may be used for the shaking. The shaking must be ofsufficient force to allow the formation of foam after a period of time.Preferably, the shaking is of sufficient force such that foam is formedwithin a short period of time, such as 30 minutes, and preferably within20 minutes, and more preferably, within 10 minutes. The shaking may beby microemulsifying, by microfluidizing, for example, swirling (such asby vortexing), side-to-side, or up and down motion. In the case of theaddition of gaseous precursor in the liquid state, sonication may beused in addition to the shaking methods set forth above. Further,different types of motion may be combined. Also, the shaking may occurby shaking the container holding the aqueous lipid solution, or byshaking the aqueous solution within the container without shaking thecontainer itself. Further, the shaking may occur manually or by machine.Mechanical shakers that may be used include, for example, a shakertable, such as a VWR Scientific (Cerritos, Calif.) shaker table, amicrofluidizer, Wig-L-Bug™ (Crescent Dental Manufacturing, Inc., Lyons,Ill.), which has been found to give particularly good results, and amechanical paint mixer, as well as other known machines. Another meansfor producing shaking includes the action of gaseous precursor emittedunder high velocity or pressure. It will also be understood thatpreferably, with a larger volume of aqueous solution, the total amountof force will be correspondingly increased. Vigorous shaking is definedas at least about 60 shaking motions per minute, and is preferred.Vortexing at least 1000 revolutions per minute, an example of vigorousshaking, is more preferred. Vortexing at 1800 revolutions per minute ismost preferred.

The formation of gaseous precursor filled microspheres upon shaking canbe detected by the presence of a foam on the top of the aqueoussolution. This is coupled with a decrease in the volume of the aqueoussolution upon the formation of foam. Preferably, the final volume of thefoam is at least about two times the initial volume of the aqueous lipidsolution; more preferably, the final volume of the foam is at leastabout three times the initial volume of the aqueous solution; even morepreferably, the final volume of the foam is at least about four timesthe initial volume of the aqueous solution; and most preferably, all ofthe aqueous lipid solution is converted to foam.

The required duration of shaking time may be determined by detection ofthe formation of foam. For example, 10 ml of lipid solution in a 50 mlcentrifuge tube may be vortexed for approximately 15-20 minutes or untilthe viscosity of the gaseous precursor-filled microspheres becomessufficiently thick so that it no longer clings to the side walls as itis swirled. At this time, the foam may cause the solution containing thegaseous precursor-filled microspheres to raise to a level of 30 to 35ml.

The concentration of stabilizing compound, especially lipid required toform a preferred foam level will vary depending upon the type ofstabilizing compound such as biocompatible lipid used, and may bereadily determined by one skilled in the art, once armed with thepresent disclosure. For example, in preferred embodiments, theconcentration of 1,2-dipalmitoylphosphatidylcholine (DPPC) used to formgaseous precursor-filled microspheres according to methods contemplatedby the present invention is about 20 mg/ml to about 30 mg/ml salinesolution. The concentration of distearoylphosphatidylcholine (DSPC) usedin preferred embodiments is about 5 mg/ml to about 10 mg/ml salinesolution.

Specifically, DPPC in a concentration of 20 mg/ml to 30 mg/ml, uponshaking, yields a total suspension and entrapped gaseous precursorvolume four times greater than the suspension volume alone. DSPC in aconcentration of 10 mg/ml, upon shaking, yields a total volumecompletely devoid of any liquid suspension volume and contains entirelyfoam.

It will be understood by one skilled in the art, once armed with thepresent disclosure, that the lipids and other stabilizing compounds usedas starting materials, or the microsphere final products, may bemanipulated prior and subsequent to being subjected to the methodscontemplated by the present invention. For example, the stabilizingcompound such as a biocompatible lipid may be hydrated and thenlyophilized, processed through freeze and thaw cycles, or simplyhydrated. In preferred embodiments, the lipid is hydrated and thenlyophilized, or hydrated, then processed through freeze and thaw cyclesand then lyophilized, prior to the formation of gaseous precursor-filledmicrospheres.

According to the methods contemplated by the present invention, thepresence of gas, such as and not limited to air, may also be provided bythe local ambient atmosphere. The local ambient atmosphere may be theatmosphere within a sealed container, or in an unsealed container, maybe the external environment. Alternatively, for example, a gas may beinjected into or otherwise added to the container having the aqueouslipid solution or into the aqueous lipid solution itself in order toprovide a gas other than air. Gases that are not heavier than air may beadded to a sealed container while gases heavier than air may be added toa sealed or an unsealed container. Accordingly, the present inventionincludes co-entrapment of air and/or other gases along with gaseousprecursors.

As already described above in the section dealing with the stabilizingcompound, the preferred methods contemplated by the present inventionare carried out at a temperature below the gel state to liquidcrystalline state phase transition temperature of the lipid employed. By"gel state to liquid crystalline state phase transition temperature", itis meant the temperature at which a lipid bilayer will convert from agel state to a liquid crystalline state. See, for example, Chapman etal., J. Biol. Chem. 1974, 249, 2512-2521.

Hence, the stabilized microsphere precursors described above, can beused in the same manner as the other stabilized microspheres used in thepresent invention, once activated by application to the tissues of ahost, where such factors as temperature or pH may be used to causegeneration of the gas. It is preferred that this embodiment is onewherein the gaseous precursors undergo phase transitions from liquid togaseous states at near the normal body temperature of said host, and arethereby activated by the temperature of said host tissues so as toundergo transition to the gaseous phase therein. More preferably still,this method is one wherein the host tissue is human tissue having anormal temperature of about 37° C., and wherein the gaseous precursorsundergo phase transitions from liquid to gaseous states near 37° C.

All of the above embodiments involving preparations of the stabilizedgas and gaseous precursor filled microspheres used in the presentinvention, may be sterilized by autoclave or sterile filtration if theseprocesses are performed before either the gas instillation step or priorto temperature mediated gas conversion of the temperature sensitivegaseous precursors within the suspension. Alternatively, one or moreanti-bactericidal agents and/or preservatives may be included in theformulation of the contrast medium, such as sodium benzoate, allquaternary ammonium salts, sodium azide, methyl paraben, propyl paraben,sorbic acid, ascorbylpalmitate, butylated hydroxyanisole, butylatedhydroxytoluene, chlorobutanol, dehydroacetic acid, ethylenediamine,monothioglycerol, potassium benzoate, potassium metabisulfite, potassiumsorbate, sodium bisulfite, sulfur dioxide, and organic mercurial salts.Such sterilization, which may also be achieved by other conventionalmeans, such as by irradiation, will be necessary where the stabilizedmicrospheres are used for imaging under invasive circumstances, e.g.,intravascularly or intraperitonealy. The appropriate means ofsterilization will be apparent to the artisan instructed by the presentdescription of the stabilized gas and gaseous precursor filledmicrospheres and their use. The contrast medium is generally stored asan aqueous suspension but in the case of dried microspheres or driedlipidic spheres the contrast medium may be stored as a dried powderready to be reconstituted prior to use.

Methods of Use

The novel stabilized gas and gaseous precursor filled microspheres,useful as contrast media in CT imaging, will be found to be suitable foruse in all areas where computed tomography imaging is employed.

In accordance with the present invention there is provided a method ofimaging a patient generally, and/or in specifically diagnosing thepresence of diseased tissue in a patient. The imaging process of thepresent invention may be carried out by administering a contrast mediumof the invention to a patient, and then scanning the patient usingcomputed tomography imaging to obtain visible images of an internalregion of a patient and/or of any diseased tissue in that region. Byregion of a patient, it is meant the whole patient or a particular areaor portion of the patient. The contrast medium is particularly useful inproviding images of the gastrointestinal region, but can also beemployed more broadly such as in imaging the vasculature or in otherways as will be readily apparent to those skilled in the art. The phrasegastrointestinal region or gastrointestinal tract, as used herein,includes the region of a patient defined by the esophagus, stomach,small and large intestines and rectum. The phrase vasculature, as usedherein, denotes the blood vessels in the body or in an organ or part ofthe body. The patient can be any type of mammal, but most preferably isa human.

As one skilled in the art would recognize, administration of thestabilized gas and gaseous precursor filled microspheres used in thepresent invention may be carried out in various fashions, such asintravascularly, orally, intrarectally, intravaginally,intravesicularly, intraperitoneally, intracochlearly,intragenitouterally, etc., using a variety of dosage forms. When theregion to be scanned is the gastrointestinal region, administration ofthe contrast medium of the invention is preferably carried out orally orrectally. The useful dosage to be administered and the particular modeof administration will vary depending upon the age, weight and theparticular mammal and region thereof to be scanned, and the particularcontrast medium of the invention to be employed. Typically, dosage isinitiated at lower levels and increased until the desired contrastenhancement is achieved. Various combinations of the stabilized gas andgaseous precursor filled microspheres may be used to alter propertiessuch as the viscosity, osmolarity or palatability, in the case of orallyadministered materials. In carrying out the CT imaging method of thepresent invention, the contrast medium can be used alone, or incombination with diagnostic, therapeutic or other agents. Such otheragents include excipients such as flavoring or coloring materials. TheCT imaging techniques which are employed are conventional and aredescribed, for example, in Computed Body Tomography, Lee, J. K. T.,Sagel, S. S., and Stanley, R. J., eds., 1983, Ravens Press, New York,N.Y., especially the first two chapters thereof entitled "PhysicalPrinciples and Instrumentation", Ter-Pogossian, M. M., and "Techniques",Aronberg, D. J.

The routes of administration and areas of usefulness of the gas andgaseous precursor filled microspheres are not limited merely to theblood volume space, i.e., the vasculature. CT imaging can be achievedwith the gas and gaseous precursor filled microspheres used in thepresent invention if the microspheres are ingested by mouth so as toimage the gastrointestinal tract. Alternatively, rectal administrationof these stabilized gas microspheres can result in excellent imaging ofthe lower gastrointestinal tract including the rectum, descending colon,transverse colon, and ascending colon, as well as the appendix. It maybe possible to achieve imaging of the ileum, and conceivably thejejunum, by way of this rectal route. As well, direct intraperitonealadministration may be achieved to visualize the peritoneum. It is alsocontemplated that the stabilized gas and gaseous precursor filledmicrospheres may be administered directly into the ear canals such thatone can visualize the canals as well as the Eustachian tubes and, if aperforation exists, the inner ear. It is also contemplated that thestabilized gas and gaseous precursor filled microspheres may beadministered intranasally to aid in the visualization of the nasalseptum as well as the nasal sinuses by computed tomography imaging.

Other routes of administration of the microsphere contrast agents of thepresent invention, and tissue areas whose imaging is enhanced therebyinclude, but are not limited to 1) intranasally for imaging the nasalpassages and sinuses including the nasal region and sinuses andsinusoids; 2) intranasally and orally for imaging the remainder of therespiratory tract, including the trachea, bronchus, bronchioles, andlungs; 3) intracochlearly for imaging the hearing passages andEustachian tubes, tympanic membranes and outer and inner ear and earcanals; 4) intraocularly for imaging the regions associated with vision;5) intraperitoneally to visualize the peritoneum; and 6)intravesicularly, i.e., through the bladder, to image all regions of thegenitourinary tract via the areas thereof, including, but not limitedto, the urethra, bladder, ureters, kidneys and renal vasculature andbeyond, e.g., to perform cystography or to confirm the presence ofureteral reflux.

The invention is further described in the following examples. All of theexamples are actual examples. These examples are for illustrativepurposes only, and are not to be construed as limiting the appendedclaims.

EXAMPLES

Various of the materials used in the following examples are commerciallyavailable. All of the lipids were purchased from Avanti Polar Lipids(Alabaster, Ala.). Perfluoropentane and perfluorohexane were purchasedfrom PCR Chemicals, Inc. (Gainesville, Fla.).

In the following examples, "DPPE" refers todipalmitoylphosphatidylethanolamine; "DPPA" refers todipalmitoylphosphatidic acid; and "DPPC" refers todipalmitoylphosphatidylcholine. "PEG 5000" refers to poly(ethyleneglycol) polymer having a molecular weight of about 5000. "DPPE-PEG-5000"refers to DPPE which is covalently bound to PEG 5000.

Example 1

This example describes the preparation of gas and gas precursor filledmicrospheres within the scope of the invention.

DPPC (77.5 mole %), DPPA (12.5 mole %), and DPPE-PEG 5000 (10 mole %)were introduced into a carrier solution of normal saline with glycerol(10 wt. %) and propylene glycol (10 wt. %). To this mixture was addedperfluoropentane and a portion of the suspension (6 mL) was placed in a18 mL glass vial and autoclaved for 15 minutes at 121° C. The resultingtranslucent suspension was allowed to cool to room temperature. Noappreciable foam could be seen was observed, but gentle shaking produceda few small bubbles at the top of the suspension. Shaking on aWig-L-Bug™ (Crescent Dental Mfg. Corp., Lyons, Ill.) for 2 minutesresulted in a dense foam that substantially filled the entire volume ofthe vial.

Samples of the lipid/perfluoropentane (PFP) suspension, with and withoutshaking, were scanned by computed tomography using a Siemens DRH SomatomIll. (Siemens, Iselin, N.J.), at 125 peak kilovolts with 410milliampseconds and an 8 millimeter slice thickness and a zoom factor of1.4. The images processed with a window width of 380 Hounsefield Units(HU) and a center of 30 HU showed fluid density in the unshaken sampleand complete blackness in the shaken sample. When examined with a windowwidth of 1,500 HU and a center of -600 HU, which corresponds to windowsettings of the type used for lung scanning, the unshaken sampleappeared bright white and the shaken sample was only faintly visible.The density of the samples was measured and the unshaken sample measured84.2 HU (S.D. 38.02) and the shaken sample measured -548.3 HU±5.92 HU.

Example 2

This example is directed to an analysis the effect of manual andmechanical shaking on microsphere size.

A lipid/PFP suspension was prepared as described in Example 1. A sampleof the suspension was shaken at room temperature manually (much lessvigorously than with the Wig-L-Bug mechanical shaker utilized in Example1). Substantially no foam was produced, only a few bubbles at the top ofthe liquid layer. However, when the sample was warmed to bodytemperature, 37° C., i.e., above the 27.5° C. boiling point of theperfluoropentane, and shaken manually, foam readily appeared and filledthe entire vial. When the foam produced by the Wig-L-Bug mechanicalshaker at room temperature was compared to the foam produced manually atbody temperature, it was noted that the microspheres produced by manualshaking were somewhat larger than the microspheres produced by theWig-L-Bug mechanical shaker. The microspheres produced by manual shakingrose to the surface more quickly than the microspheres produced by theWig-L-Bug mechanical shaker, a further indication that the microspheresproduced by mechanical shaking were smaller than the microspheresproduced by manual shaking, since larger microspheres will rise morequickly.

Example 3

This example is directed to the formation of stabilized gas-filledmicrospheres comprising lipid bilayers with polyvinyl alcohol.

The effect of a polymer, namely, polyvinylalcohol, on the size ofmicrospheres containing perfluorocarbons is illustrated in this example.Gas-filled microspheres comprising a lipid were prepared by the additionof 5 mg/mL of a suspension of DPPC:phosphatidic acid, and DPPE-PEG 5000in a molar weight ratio of 82:8:10 in a vehicle containing 5% by weightof polyvinylalcohol (weight average M.W. 50,000, 99+% hydrolyzed) innormal saline. To this mixture was added 50 μL of perfluoropentane. Anidentical suspension to the above described suspension was also preparedexcept that the vehicle was normal saline, glycerol, and propyleneglycol in a ratio of 8:1:1, v:v:v (Spectrum Chemical Co., Gardena,Calif.). The suspensions were then autoclaved at 121° C. for 21 minutesin a Barnstead/Thermolyne autoclave (Barnstead/Thermolyne, RanchoDominguez, Calif). The temperatures of the resultant products were thenequilibrated to 30° C. in a VWR Model 2500 incubator (VWR ManufacturingCorp., Albuquerque, N.Mex.). The slightly opaque suspensions were thenshaken on a Wig-L-Bug shaker (Crescent Dental Mfg. Corp., Lyons, Ill.)for two minutes. This led to the production of foams. The subsequentfoam samples were then sized on a Particle Sizing Systems Model 770light obscuration sizer. The instrument was calibrated with standardsized latex beads ranging in size from 2.02 μm to 41.55 μm (CoulterElectronics, Inc., Hialeah, Fla.). The sampling vehicle was deionizedwater. The size distributions of the PVA-containing sample vs. thenormal saline, glycerol, propylene glycol sample were as follows:

                  TABLE 3                                                         ______________________________________                                        Sizing of Gas-Filled Microspheres Comprising                                  Lipid Bilayers with and without Polyvinyl Alcohol (PVA)                                            Normal Saline,                                                        5%      Glycerol, Propylene                                                   PVA Sample                                                                            Glycol Sample                                            ______________________________________                                        Average Size   5.51 μm                                                                              5.82 μm                                           95%     less than  14.45 μm                                                                             19.1 μm                                       99.9%   less than  72.2 μm                                                                              75.6 μm                                       ______________________________________                                    

Example 4

This example describes the use of perfluoropentane in the preparation ofmicrospheres comprising lipid bilayers.

In an 18 mL vial, 6 mL of a suspension of 5 mg/mL lipid consisting of77.5 mole % 1,2 dipalmitoyl-3-sn glycerophosphatidylcholine (DPPC), 12.5mole % phosphatidic acid, and 10 mole % 1,2dipalmitoyl-3-sn-phosphatidylethanolamine-polyethyleneglycol 5000(DPPE-PEG 5000) was added followed by the addition of 50 μL ofperfluoropentane, injected into the solution at room temperature. Thehead space in the vial was filled with air at ambient pressure and thevial was sealed with a teflon stopper and aluminum seal (VWR,Albuquerque, N. Mex.). The vial was then autoclaved at 121° C. for 15minutes (Barnstead Thermolyne, Dubuque, Iowa). A translucent homogeneoussuspension resulted. The vial was then removed from the autoclave,allowed to cool to room temperature, and then shaken for two minutes ona Wig-L-Bug shaker (Crescent Dental Manufacturing Corp., Lyons, Ill.).The entire vial was then found to be filled with foam. The vial wasthereafter placed in a refrigerator at 4° C. and the foam persisted. Bycomparison, foam prepared with air or nitrogen gas alone in the samemixture of lipids, i.e., without the addition of perfluoropentane, didnot persist as long as the foam produced using perfluoropentane. Theduration of the foam prepared with the mixture of perfluoropentane andair was surprising; particularly so when it is considered that theboiling point of perfluoropentane is approximately 27.5° C. Roomtemperature under the conditions of this experiment was about 20° C.,and thus at 4° C. it would have expected that the foam would collapse.This experiment thus demonstrates the surprising discovery that thepresence of perfluorocarbons, despite being in the liquid state, cancontribute to stabilization of the foam.

Example 5

This example describes the use of perfluorohexane in the preparation ofmicrospheres comprising lipid bilayers.

To further demonstrate that a liquid-state perfluorocarbon cancontribute to stabilization of a gas-filled microsphere foam, the resultobtained using perfluorohexane (b.p. 56° C.) was evaluated. A suspensionof lipids was prepared as described above in Example 4, except that 50μL of perfluorohexane was added to the vial in lieu of theperfluoropentane. The suspension was autoclaved yielding atranslucent-to-cloudy suspension of lipids. The material was shaken onthe Wig-L-Bug (Crescent Dental Mfg. Corp., Lyons, Ill.) for two minutesand a foam again resulted. The volume of foam was greater than thecontrol sample, which utilized air alone as the ambient gas. Once again,the foam remained stable and persisted longer than the air sample. Thisclearly demonstrates that the presence of the perfluorohexane, which isliquid at human physiological temperatures, functions to stabilize agas-filled microsphere foam.

Example 6

This example describes trehalose stabilization of gas-filledmicrospheres comprising lipid bilayers.

A gas-filled microsphere foam was prepared from a stabilizing compoundvehicle comprising normal saline:glycerol:propylene glycol (8:1:1,v:v:v) with the lipids set forth in Example 4 above, and shaken asdescribed therein on a Wig-L-Bug for two minutes, yielding approximately6 mL of foam at room temperature. After four days, it was discoveredthat the foam was no longer present. When the above experiment wasrepeated with the same lipids, except that trehalose, D-glucopyranose, adisaccharide, was added in a 1:1 molar ratio of trehalose to lipid, thefoam was found to persist longer than the control. Repeating theexperiment yielded similar results. This experiment clearly demonstratesthat trehalose can function as an auxiliary stabilizing compound tolengthen the time duration of gas-filled microspheres of the presentinvention.

The disclosures of each patent, patent application and publication citedor described in this document are hereby incorporated by reference, intheir entirety.

Various modifications of the invention, in addition to those describedherein, will be apparent to those skilled in the art from the foregoingdescription. Such modifications are also intended to fall within thescope of the appended claims.

What is claimed is:
 1. A method of providing an image of an internalregion of a patient comprising (i) administering to the patient acomposition comprising, in an aqueous carrier, gas-filled microspheres,wherein said gas comprises a fluorinated gas selected from the groupconsisting of perfluorocarbons and sulfur hexafluoride, and saidmicrospheres comprise a biocompatible lipid, bearing a hydrophilicpolymer, said hydrophilic polymer covalently bound to said biocompatiblelipid through an amide, carbamate, amine, ester, ether thioester orthioamide linkage, and (ii) scanning the patient using computedtomography to obtain visible images of the region.
 2. A method accordingto claim 1 wherein the region comprises the vasculature.
 3. A methodaccording to claim 1 wherein the region comprises the cardiovascularregion.
 4. A method according to claim 1 wherein the region comprisesthe gastrointestinal region.
 5. A method for diagnosing the presence ofdiseased tissue in a patient comprising (i) administering to the patienta composition comprising, in an aqueous carrier, gas-filledmicrospheres, wherein said gas comprises a fluorinated gas selected fromthe group consisting of perfluorocarbons and sulfur hexafluoride, andsaid microspheres comprise a biocompatible lipid bearing a hydrophilicpolymer, said hydrophilic polymer covalently bound to said biocompatiblelipid through an amide, carbamate, amine, ester, ether, thioester orthioamide region.
 6. A method according to claim 5 wherein the regioncomprises the vasculature.
 7. A method according to claim 5 wherein theregion comprises the cardiovascular region.
 8. A method according toclaim 5 wherein the region comprises the gastrointestinal region.
 9. Amethod according to claim 5 wherein said scanning is of a region of apatient selected from the group consisting of the intranasal tract, theauditory canal, the intraocular region, the intraperitoneal region, thekidneys, the urethra and the genitourinary tract.